This replacement is a little menial, but it explicitly catches all
the places where we allow a local socket. The actual implementation of
opening a AF_UNIX socket is almost hidden in the patch.
The detection of "valid address" is now more complex:
p->addr.itype != ADDR_INTERNAL_WIREADDR || p->addr.u.wireaddr.type != ADDR_TYPE_PADDING
But most places we do this, we should audit: I'm pretty sure we can't
get an invalid address any more from gossipd (they may be in db, but
we should fix that too).
Closes: #1323
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>