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.

lib/io.c

@@ -7,7 +7,7 @@
 
 #include <string.h>
 #include <errno.h>
-#include <asm/types.h>
+#include <linux/types.h>
 
 void check_memory_msg(void)
 {
@@ -585,8 +585,8 @@ static void erase_rollback_file(char *rollback_file)
 
 int open_rollback_file(char *rollback_file, FILE * log)
 {
-	char string[28];
 	struct stat buf;
+	char string[sizeof(ROLLBACK_FILE_START_MAGIC)] = { 0 };
 
 	if (rollback_file == NULL)
 		return -1;
@@ -603,8 +603,8 @@ int open_rollback_file(char *rollback_file, FILE * log)
 		return -1;
 	}
 
-	fread(string, 28, 1, s_rollback_file);
-	if (!strcmp(string, ROLLBACK_FILE_START_MAGIC)) {
+	fread(string, sizeof(ROLLBACK_FILE_START_MAGIC) - 1, 1, s_rollback_file);
+	if (strcmp(string, ROLLBACK_FILE_START_MAGIC) != 0) {
 		fprintf(stderr,
 			"Specified file (%s) does not look like a rollback file\n",
 			rollback_file);
@@ -775,8 +775,9 @@ void do_fsck_rollback(int fd_device, int fd_journal_device, FILE * progress)
 
 	printf("\n");
 	if (log_file)
-		fprintf(log_file, "rollback: (%u) blocks restored\n",
-			count_rollbacked);
+		fprintf(log_file,
+			"rollback: (%u) blocks restored, %u skipped\n",
+			count_rollbacked, count_failed);
 }
 
 /*