nfs-utils: Move common code into support

There are several source files and headers present in the ./utils/idmapd
directory which are also usable in a doimapd daemon. Because of this we
move that support into the support directory such that it can be shared by
both daemons.

Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>