basic/socket-util: use c-escaping to print unprintable socket paths

We are pretty careful to reject abstract sockets that are too long to fit in
the address structure as a NUL-terminated string. And since we parse sockets as
strings, it is not possible to embed a NUL in the the address either.  But we
might receive an external socket (abstract or not), and we want to be able to
print its address in all cases. We would call socket_address_verify() and
refuse to print various sockets that the kernel considers legit.

Let's do the strict verification only in case of socket addresses we parse and
open ourselves, and do less strict verification when printing addresses of
existing sockets, and use c-escaping to print embedded NULs and such.

More tests are added.

This should make LGTM happier because on FIXME comment is removed.

src/basic/socket-label.c

@@ -39,7 +39,7 @@ int socket_address_listen(
 
         assert(a);
 
-        r = socket_address_verify(a);
+        r = socket_address_verify(a, true);
         if (r < 0)
                 return r;