libexport.a: Reduce code duplication in client_init()

Clean up:  Most cases in client_init() set clp->m_naddr to zero.  Move
it to the common part of the function, and simplify the logic.  This
will make adding IPv6 support here more straightforward.

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