Additional future features for socket-activate(1)

* abstract unix sockets (not in the filesystem)

* bind to specific network interfaces instead of (or in addition to)
  addresses?

* SCTP / "Sequential packet"

* other socket options (SO_PRIORITY, SO_RCVBUF, SO_SNDBUF, SO_REUSEPORT, etc)

* reimplement in C?

* add unlink option for Unix-domain sockets in case the socket already exists

* improve tests/basic to cover TCP, UDP, Unix-domain datagrams, and all the options.

* improve tests/basic to cover when the intended file descriptors are already active

* notify the invoker somehow when sockets are successfully all open
  (daemonization is one possible way, but i'd rather not encourage
  daemonization)

* have an option to not select() before exec()ing.

* add a test that uses an actual socket-activated daemon

* other forms of socket activation besides inet and unix-domain, like
  inotify, etc (might be linux-specific)

* write clear advocacy that encourages daemon authors to avoid opening
  sockets, daemonization, privilege-dropping, etc themselves, since
  they're available (and someone else's problem) without any extra code.

* environment manipulation?
