Make capabilities support optional

The new code using libcap is quite minor, so rather than always reqiure
libcap support, make it a normal --enable type flag.  Current default
behavior is retained -- if libcap is found, it is enabled, else it is
disabled like every nfs-utils version in the past.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Steve Dickson <steved@redhat.com>