support: Provide an API for creating a privileged RPC client

We needed to guarantee that some RPC programs, such as PMAP, got an
unprivileged port, to prevent exhausting the local privileged port
space sending RPC requests that don't need such privileges.
nfs_get_rpcclient() provides that feature.

However, some RPC programs, such as MNT and UMNT, require a privileged
port.  So, let's provide an additional API for this that also supports
IPv6 and setting a destination port.

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