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.

fsck/pass2.c

@@ -4,6 +4,7 @@
  */
 
 #include "fsck.h"
+#include "misc.h"
 
 /* on pass2 we take leaves which could not be inserted into tree
    during pass1 and insert each item separately. It is possible that
@@ -74,7 +75,7 @@ static __u64 link_one(struct relocated *file)
 	struct reiserfs_key obj_key;
 	__u64 len = 0;
 
-	asprintf(&name, "%lu,%lu", file->old_dir_id, file->new_objectid);
+	xasprintf(&name, "%lu,%lu", file->old_dir_id, file->new_objectid);
 	set_key_dirid(&obj_key, file->old_dir_id);
 	set_key_objectid(&obj_key, file->new_objectid);
 
@@ -118,15 +119,12 @@ void linked_already(const struct reiserfs_key *new_key /*, link_func_t link_func
 void link_relocated_files(void)
 {
 	struct relocated *tmp;
-	int count;
 
-	count = 0;
 	while (relocated_list) {
 		link_one(relocated_list);
 		tmp = relocated_list;
 		relocated_list = relocated_list->next;
 		freemem(tmp);
-		count++;
 	}
 }
 
@@ -369,7 +367,7 @@ static void put_directory_item_into_tree(struct item_head *comingih, char *item)
 			     " badly hashed entry", i, namelen, name,
 			     &comingih->ih_key);
 
-		asprintf(&buf, "%.*s", namelen, name);
+		xasprintf(&buf, "%.*s", namelen, name);
 		/* 1 for fsck is important: if there is no any items of this
 		   directory in the tree yet - new item will be inserted
 		   marked not reached */