showmount command: call nfs_getport instead of local getport

Have the showmount command invoke the shared nfs_getport() function
instead of its own local version.  This gives the showmount command
immediate support for querying via rpcbindv3/v4 in addition to
portmapper, and sets the stage for AF_INET6 support in showmount.

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