libexport.a: IPv6 support for client_init_subnet()

To parse and store an IPv6 host or subnet address, init_netmask()
needs to handle 128 bit subnet masks.

Unfortunately what once was a pretty simple little function has grown
much larger.  This logic must now not only parse IPv6 addresses
correctly, but must also distinguish between IPv4 and IPv6.

To avoid code duplication, I'm "bending" the cardinal rule of not
using "#ifdef" inside functions.

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