Systemd, when doing socket activation, pass file descriptors in a
non-stable order when there is multiples ones.
But we only use one, so we don't need to identify it, and the file
descriptors always start at 3.
So use 3 for the systemd service.
Source : sd_listen_fds (systemd man pages)
We also need to unset variables systemd pass to socket activated
process, since we don't need them and sub-process (footclient and
theirs forks) could be confused by those.
Closes#1107
- Support for multiples concurrent instances, templated on
WAYLAND_DISPLAY
- Use standard input for the socket file descriptor (inetd style)
- Always use the socket provided by systemd with the systemd user
service
- wayland-instance@.target is intended to be a special target a bit like
graphical-session.target.
