configure: pull common nfsidmap and event checks into aclocal/

Clean up: Create an aclocal script for the nfsidmap library and
headers checks used for both --enable-gss and --enable-nfsv4.
Move libevent checks out too.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>