If the target file already exists, check for the CAP_UNLINKAT capabiity right
on the target directory descriptor, but only if this is renameat(2) and real
target directory descriptor is given (not AT_FDCWD). Without this fix regular
rename(2) fails if the target file already exists.

Reported by:	Michael Butler <imb@protected-networks.net>
Reported by:	Larry Rosenman <ler@lerctr.org>
Sponsored by:	The FreeBSD Foundation