promise/plugin: Add check_socket_listening promise

This promise generalizes the check_port_listening promise to include
AF_UNIX sockets in addition to AF_INET and AF_INET6 host/port sockets.

The socket family and address is deduced from the set of arguments:
- `host` & `port`: AF_INET or AF_INET6
- `pathname`: filesystem pathname address for AF_UNIX
- `abstract`: '\0' is prefixed to build an abstract AF_UNIX address

All other combinations of arguments are forbidden.

The promise creates a socket accordingly and attempts to connect to the
provided address. The promise fails if the connection fails.

The check_port_listening promise has been removed.

See merge request !97