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.

debugreiserfs/scan.c

@@ -4,6 +4,7 @@
  */
 
 #include "debugreiserfs.h"
+#include "misc.h"
 #include <regex.h>
 #include <obstack.h>
 #include <search.h>
@@ -253,7 +254,7 @@ static struct saved_name *scan_for_key(const struct reiserfs_key *key)
 	char *name;
 	struct saved_name *new, *name_in;
 
-	asprintf(&name, "%u_%u", get_key_dirid(key), get_key_objectid(key));
+	xasprintf(&name, "%u_%u", get_key_dirid(key), get_key_objectid(key));
 	new =
 	    obstack_alloc(&name_store,
 			  sizeof(struct saved_name) + strlen(name));
@@ -591,7 +592,7 @@ static void flush_map(reiserfs_filsys_t fs,
 	__u32 v32;
 
 	if (!map_file(fs))
-		asprintf(&map_file(fs), "%s", ".map");
+		xasprintf(&map_file(fs), "%s", ".map");
 
 	//reiserfs_warning (stderr, "Saving maps into %s\n", map_file (fs));
 	fp = fopen(map_file(fs), "a");
@@ -814,7 +815,7 @@ static void make_map(const void *nodep, VISIT value, int level)
 	if (value == leaf || value == postorder) {
 		while (name) {
 			if (map_file(fs)) {
-				asprintf(&file_name, "%s.%d", map_file(fs),
+				xasprintf(&file_name, "%s.%d", map_file(fs),
 					 ++nr);
 				reiserfs_warning(log_to,
 						 "%d - (%d): [%K]:\"%s\": stored in the %s\n",
@@ -1006,7 +1007,7 @@ void do_scan(reiserfs_filsys_t fs)
 	int type;
 	char *answer = 0;
 	size_t n = 0;
-	struct reiserfs_key key = { 0, 0, };
+	struct reiserfs_key key = { 0 };
 	unsigned long done, total;
 
 	if (debug_mode(fs) == DO_LOOK_FOR_NAME) {