fchdir: simplify error handling, and support dup3

fchdir: simplify error handling, and support dup3

* modules/fchdir (Depends-on): Use strdup-posix, not strdup.  Add
stdbool, malloc-posix, realloc-posix.
* lib/fchdir.c (struct dir_info_t): Delete saved_errno.
(ensure_dirs_slot): Return false on allocation failure.
(rpl_dup2): Delete.
(_gl_register_dup): New function.
(_gl_unregister_fd, rpl_opendir, rpl_dup): Update callers.
(_gl_register_fd): Close fd on allocation failure.
* lib/fcntl.in.h (_gl_register_fd): Update signature.
* lib/unistd.in.h (_gl_register_dup) [FCHDIR_REPLACEMENT]: New
prototype.
(rpl_dup2_fchdir): Delete prototype.
* lib/open.c (open): Update caller.
* lib/dup2.c (dup2): Track fchdir metadata.
* lib/dup3.c (dup3): Likewise.
* m4/dup2.m4 (gl_REPLACE_DUP2): New macro.
* m4/fchdir.m4 (gl_FUNC_FCHDIR): Use it.

Signed-off-by: Eric Blake <ebb9@byu.net>