Introduce rpcbind client utility functions

Introduce rpcbind client utility functions

It turns out that at least the mount command and the showmount command
need to query a server's rpcbind daemon.  They need to query over
AF_INET6 as well as AF_INET.

libtirpc provides an rpcbind query capability with the rpcb_getaddr(3)
interface, but it takes a hostname and netconfig entry rather than a
sockaddr and a protocol type, and always uses a lengthy timeout.  The
former is important to the mount command because it sometimes must
operate using a specific port and IP address rather than depending on
rpcbind and DNS to convert a [hostname, RPC program, netconfig] tuple
to a [socket address, port number, transport protocol] tuple.

The rpcb_getaddr(3) API also always uses a privileged port (at least
for setuid root executables like mount.nfs), which is not required for
an rpcbind query.  This can exhaust the local system's reserved port
space quickly.

This patch provides a reserved-port-friendly AF_INET6-capable rpcbind
query C API that can be shared among commands and tools in nfs-utils,
and allows a query to a specified socket address and port rather than
a hostname.

In addition to an rpcbind query interface, this patch also provides a
facility to ping the remote RPC service to ensure that it is operating
as advertised by rpcbind.  It's useful to combine an RPC ping with an
rpcbind query because in many cases, components of nfs-utils already
ping an RPC service immediately after receiving a successful GETPORT
result.

There are also a handful of utility routines provided, such as a
functions that can map between [sockaddr, port] and a universal
address.

I've made an attempt to make these new functions build and operate on
systems that do not have libtirpc.

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