mount command: AF_INET6 support for probe_bothports()

Introduce an AF_INET6 capable probe_bothports() API.  This means replacing
"struct sockaddr_in *" arguments with a "struct sockaddr *" and a socklen_t
arguments.

These functions often combine a "struct sockaddr_in" and a "struct pmap" into
a single "clnt_addr_t" argument.  Instead of modifying "clnt_addr_t" and all
the legacy code that uses it, I'm going to create a new probe_bothports() API
for the text-based mount command that takes a "struct sockaddr *" and
sockaddr length, and leave the existing probe_bothports() interface, which
takes "clnt_addr_t" arguments, for legacy use.

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