libexport.a: Prepare to recognize IPv6 addresses in client_gettype()

The current open-coded parsing logic in client_gettype() will be hard
to modify to recognize IPv6 addresses.  Use a more generic mechanism
for detecting IP presentation addresses.

IPv6 will be enabled automatically in client_gettype() when host_pton()
is changed to support IPv6 addresses.

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