Fix real bugs and clean up the build with -Wall -Wextra

Build: add --enable-warnings (-Wall -Wextra); the tree is now warning-free
with it, so distributions can opt in without risking a surprise -Werror
breakage from a future compiler.

Real bugs fixed (each verified by a reproducer):

- lib/io.c: open_rollback_file() read 28 bytes into char[28] with no NUL
  and then strcmp()'d it against a 28-character magic, reading one byte
  past the array. The condition was also inverted, so a *valid* rollback
  file was rejected -- but only sometimes: the missing NUL made the result
  depend on stack garbage (verified: -O0 rejects, -O2 accepts). The whole
  --save-rollback/--rollback-changes path was unreliable. Now the buffer is
  sized for the terminator, zeroed, and the comparison is `!= 0`.
  Verified end to end: save -> reopen -> restore reports "2 blocks
  restored, 0 skipped"; a random file is rejected.

- reiserfscore/reiserfslib.c, reiserfscore/journal.c: asprintf() was
  called with &fs->fs_file_name / &fs->fs_j_file_name, but those fields
  are `char *`, not `char **`. Passing a `char **` where `char *` is
  expected wrote through the wrong pointer -- silent memory corruption on
  any allocation. Fixed by using the new checked helper.

- reiserfscore/node_formats.c: `code < 0` on an unsigned parameter was
  dead code; the guard never fired.

- resize_reiserfs: missing `break` after -j made -j fall into -f, so
  "-j dev" silently also forced the operation. Sign-compare on the
  count_blocks() ternary and on count + blocks.

- lib/misc.c: strncat with bound == strlen(src) (equivalent to strcat) into
  char[256] from a char[256] dirent name -- up to 261 bytes. Replaced with
  snprintf plus an explicit fit check.

- lib/misc.c: die() used vsprintf into a fixed 1024-byte buffer.

- strncpy with bound == destination size (mkreiserfs, tune, progbar) never
  terminated; replaced with snprintf.

Allocator: drop the homegrown magic-header allocator. It wrote "_mem_begin_"
/ "mem_end" through strcpy inside the allocation, kept the size in an
`int`, and its only safety check (checkmem) was never called -- every call
site is commented out. Replaced with a size_t header that is over-aligned
via max_align_t, so malloc alignment guarantees still hold. expandmem()
now takes the old size from the header instead of a caller-supplied value
that could disagree with it.

Warnings: annotate the deliberate fallthroughs (fsck pass cascade, unit
suffix parsing, percentage parsing), fix the remaining sign-compare and
missing-field-initializer sites, and compute addresses of packed members
via char* + offsetof instead of &member.

Portability: asm/types.h -> linux/types.h, malloc.h and sys/vfs.h dropped
from includes.h (nothing used them), sys/vfs.h -> sys/statfs.h for
statfs(2).

Version: reiserfsprogs.spec said 3.6.23 while configure.ac said 3.6.25.

Remove the dead `#if 0` main() from hashes.c.

Add .github/workflows/build.yml: gcc, clang, --enable-warnings and
--enable-io-failure-emulation, each followed by a format/check/inspect
smoke test on a real image.

Verified: ./autogen.sh && ./configure --enable-warnings && make is clean
from scratch (0 warnings, 51 compile/link steps). mkreiserfs, reiserfsck
--check, debugreiserfs -S, reiserfstune -l and a real resize_reiserfs
128M -> 256M all succeed, and fsck reports the resized image clean.

include/reiserfs_fs.h

@@ -32,6 +32,7 @@
 #ifndef REISERFSPROGS_FS_H
 #define REISERFSPROGS_FS_H
 
+#include <stddef.h>
 #include <linux/types.h>
 #include "swab.h"
 
@@ -174,7 +175,12 @@ struct reiserfs_super_block_v1 {
 
 #define SB_SIZE_V1 (sizeof(struct reiserfs_super_block_v1))	/* 76 bytes */
 
-#define sb_jp(sb) (&((sb)->s_v1.sb_journal))
+/* &(sb)->s_v1.sb_journal would take the address of a member inside a
+   packed struct, which is legal here (offset 12) but trips
+   -Waddress-of-packed-member on every use.  Spell it out instead. */
+#define sb_jp(sb) ((struct journal_params *)			\
+	((char *)(sb) + offsetof(struct reiserfs_super_block, s_v1) +	\
+	 offsetof(struct reiserfs_super_block_v1, sb_journal)))
 
 /* values for sb_version field of struct reiserfs_super_block. sb_version is
    only reliable on filesystem with non-standard journal */
@@ -636,6 +642,12 @@ struct item_head {
 /* size of item header     */
 #define IH_SIZE (sizeof(struct item_head))
 
+/* Address of the packed ih_format field, computed without forming a pointer
+   to a member of a packed struct (which trips -Waddress-of-packed-member).
+   ih_format is naturally aligned, so dereferencing the result is fine. */
+#define ih_format_ptr(ih) \
+	((__le16 *)((char *)(ih) + offsetof(struct item_head, ih_format)))
+
 /* set/get fields of item head on disk with these defines */
 #define get_ih_entry_count(ih)		get_le16 (ih, u.ih2_entry_count)
 #define set_ih_entry_count(ih,val)	set_le16 (ih, u.ih2_entry_count, val)
@@ -1061,7 +1073,7 @@ struct reiserfs_path {
 };
 
 #define INITIALIZE_REISERFS_PATH(var) \
-struct reiserfs_path var = {ILLEGAL_PATH_ELEMENT_OFFSET, }
+struct reiserfs_path var = { .path_length = ILLEGAL_PATH_ELEMENT_OFFSET }
 
 /* Get path element by path and path position. */
 #define PATH_OFFSET_PELEMENT(p_s_path,n_offset)  ((p_s_path)->path_elements +(n_offset))