drop systemd socket activation support

Replace systemd socket activation with old-style network interface configuration in config file.

CAP_NET_BIND_SERVICE should be added during service startup and dropped once sockets are bound via net.listen()

For more details, see discussion on mailing list: https://lists.nic.cz/pipermail/knot-resolver-users/2019/000182.html

Related: #484 (closed) #342 (closed) #445 (closed)

Related changes

(preliminary plans)

• failing net.listen() should throw a lua error and therefore fail kresd if specified in configuration (by default)
• allow specifying FREEBIND in net.listen()
• unify TTY sockets, e.g. net.listen('path', nil, { kind = 'control' })
• add distro-specific preconfig (control socket location, cache size)
• use upgrade script to suggest updates to config and test in various envs