SUNRPC: support abstract unix socket addresses

An "abtract" address for an AF_UNIX socket start with a nul and can
contain any bytes for the given length, but traditionally doesn't
contain other nuls.  When reported, the leading nul is replaced by '@'.

sunrpc currently rejects connections to these addresses and reports them
as an empty string.  To provide support for future use of these
addresses, allow them for outgoing connections and report them more
usefully.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>