dirent: Use an actual flexible-length array for d_name.

GCC 11 treats [1] as a fixed-length array and not a flexible-length
array, and so some things that used direct strcmp("..", ent->d_name),
for instance, would be optimized out as being always unequal,
which was the cause of #17389. Using a real FLA informs GCC that
there is going to be more than one byte of data, and thus this
fixes that bug.

BeOS used [1] and not [0], possibly because it had to deal with
compilers (MetroWerks? Early GCC2?) that did not support FLAs.
GCC 2.95 does, using [0], and GCC 4 does, using [], so we can go
with that here.

(I did try using [0] for both, which seems to be OK with GCC 11,
but GCC 8 throws errors when d_name is dereferenced directly
as being-out-of-bounds. So, we have to use the #if here and give
newer GCC the [] syntax and not [0] to avoid that problem.)

The real question probably is whether or not we should backport
some variant of these changes to R1/beta3, as software at HaikuPorts
very well may run in to the same issue. (The alternative workaround
is to compile with -O1 and not -O2 for any affected software.) But
maybe this is an argument for keeping with the beta4 schedule of
this coming January...

headers/posix/dirent.h

@@ -15,7 +15,11 @@ typedef struct dirent {
 	ino_t			d_ino;		/* inode number */
 	ino_t			d_pino;		/* parent inode (only for queries) */
 	unsigned short	d_reclen;	/* length of this record, not the name */
-	char			d_name[1];	/* name of the entry (null byte terminated) */
+#if __GNUC__ == 2
+	char			d_name[0];	/* name of the entry (null byte terminated) */
+#else
+	char			d_name[];	/* name of the entry (null byte terminated) */
+#endif
 } dirent_t;
 
 typedef struct __DIR DIR;

headers/private/fs_shell/fssh_dirent.h

@@ -14,7 +14,11 @@ typedef struct fssh_dirent {
 	fssh_ino_t			d_ino;		/* inode number */
 	fssh_ino_t			d_pino;		/* parent inode (only for queries) */
 	unsigned short		d_reclen;	/* length of this record, not the name */
-	char				d_name[1];	/* name of the entry (null byte terminated) */
+#if __GNUC__ == 2
+	char			d_name[0];	/* name of the entry (null byte terminated) */
+#else
+	char			d_name[];	/* name of the entry (null byte terminated) */
+#endif
 } fssh_dirent_t;
 
 typedef struct {