Forced shutdown timeout from configuration file

[bioxz]

This change allows to define the timeout for a forced shutdown in the configuration file. Before this change, the server was killed after a fixed 15 second timeout, which is to small for bigger setups (e.g. multiple map mods like Space Exploration) or slow hardware. Default value in configuration file and if not configured is 15 (the old value).

If changed, the systemd service file must be adjusted as well. This is noted in a comment.

Fixes #191.