HTTP: reuse socket from systemd if provided, allowing for systemd socket activation

[NickHu]

Systemd can start services on-demand when a socket is polled. This is done as
follows:

1. systemd listens on that socket from boot;
2. when the socket first receives data:
   1. systemd starts the configured service (in this case, mopidy);
   2. systemd hands the socket over to the configured service.

The last part requires the application to support this, which is what this PR
implements.

Here is the contents of my ~/.config/systemd/user/mopidy.socket:

[Install]
WantedBy=sockets.target

[Socket]
Backlog=5
FileDescriptorName=http
KeepAlive=true
ListenStream=127.0.0.1:6680

(It starts mopidy.service).

[jodal]

Same feedback here as in Mopidy-MPD: The code would be cleaner with a get_systemd_socket() -> Optional[socket] function.