showmount command: move logic to acquire RPC client handle out of main()

In preparation to support IPv6 in the showmount command, extract the
logic that parses/acquires the target hostname and converts it into an RPC
client handle to contact the remote mountd service, and move it into its
own function.

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