(OPENAT_CWD_RESTORE__REQUIRE): Remove.

(OPENAT_CWD_RESTORE__REQUIRE): Remove.
(OPENAT_CWD_RESTORE__ALLOW_FAILURE): Likewise.
(fd_to_subdirp): Remove openat_cwd_restore_allow_failure arg; its
value is now signified by whether cwd_errno is null.
(fd_to_subdirp, remove_dir, rm_1); Change cwd failure indicator from
pointer-to-bool to pointer-to-errno-value.  All callers changed.
(rm_1): Don't bother setting a local cwd failure flag and then
ORing it into the caller's.  Just set the caller's.
(rm): Use cwd failure errno value to print a slightly-better
diagnostic.