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_lib.h

@@ -172,17 +172,17 @@ typedef int (*reiserfs_file_iterate_direct_fn)(reiserfs_filsys_t fs,
 					       const char *body, size_t len,
 					       void *data);
 int reiserfs_iterate_file_data(reiserfs_filsys_t fs,
-			       const struct reiserfs_key const *short_key,
+			       const struct reiserfs_key *short_key,
 			       reiserfs_file_iterate_indirect_fn indirect_fn,
 			       reiserfs_file_iterate_direct_fn direct_fn,
 			       void *data);
 
 typedef int (*reiserfs_iterate_dir_fn)(reiserfs_filsys_t fs,
-		       const struct reiserfs_key const *dir_short_key,
+		       const struct reiserfs_key *dir_short_key,
 		       const char *name, size_t len,
 		       __u32 deh_dirid, __u32 deh_objectid, void *data);
 int reiserfs_iterate_dir(reiserfs_filsys_t fs,
-			 const struct reiserfs_key const *dir_short_key,
+			 const struct reiserfs_key *dir_short_key,
 			 const reiserfs_iterate_dir_fn callback, void *data);