Write a helper library for services/daemons

A very common use case for a library like trio is writing long-running server processes. We should make it easy to get this right. This should probably be as a new package on top of trio. Some brainstorming:

Integrating with platform-specific service management systems:

• Windows services

• Launchd integration, in particular socket activation. This is done by calling launch_activate_socket("lookup-key", ...), which returns an array of fds. (Could be multiple e.g. one for IPv4 and one for IPv6).

• Systemd integration: socket activation (depends on #251), watchdog support, sd_notify, maybe sd_pid_notify_with_fds, ... (see sd-daemon.h). These can be done via a binding to libsystemd or through our own implementation (they're all pretty trivially defined in terms of environment variable manipulation etc.), and either way there's a choice whether to write our own or use an existing one.

It might also make sense to support classic double-fork daemonization, though that's fallen out of popularity these days.

Generic services:

• abstracting away the differences between different socket activation styles (a bit tricky, since launchd always uses names but we can't really introspect the returned sockets – again see #251 – while systemd prefers to use introspection and only supports names as a fallback.)

• graceful shutdown -- SIGTERM, however windows services do it

Start with the proposed entry points: launch_activate_socket, the systemd sd-daemon.h interface, and dependency issue #251. Done would require defining a package scope that covers the relevant service-management integrations, socket activation differences, and graceful shutdown behavior.