Change AT_FDCWD from -1 to -100.

Change the value of `AT_FDCWD` from -1 to -100. Either value should work,
since the Haiku implementation currently interprets any negative value as
`AT_FDCWD`, however -100 is preferable to -1 since -1 is used for error
return values.

fixes #18809

Change-Id: I53f273dd595c39ea9e4576b90adbbf7f555107cf
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7423
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

headers/posix/fcntl.h

@@ -59,7 +59,7 @@
 #define O_DIRECTORY		0x00200000	/* fail if not a directory */
 
 /* flags for the *at() functions */
-#define AT_FDCWD		(-1)		/* CWD FD for the *at() functions */
+#define AT_FDCWD		(-100)		/* CWD FD for the *at() functions */
 
 #define AT_SYMLINK_NOFOLLOW	0x01	/* fstatat(), fchmodat(), fchownat(),
 										utimensat() */