New configuration option "IdleTimeout": exit daemon when idle

New configuration option "IdleTimeout": exit daemon when idle

This patch implements a new configuration option "IdleTimeout" in the
[Limits] section of the configuration file which can be used to set a
timeout (in seconds) after which the whole daemon will shutdown when no
more connections are left active after handling at least one client.

The default is 0, "never".

This can be useful for testing or when ngIRCd is started using "socket
activation" with systemd(8), for example.