[SERVER] Support for custom actions on different signals (SIGHUP, SIGINT, etc)

[keksbg]

I think this title should be self-explanatory, I'm curious if it'd be viable to create support for custom actions for the different signals that can be sent to a process (like SIGHUP, SIGINT, SIGKILL, SIGQUIT) and possibly also making it scriptable. As a good stepping stone, it could be a good idea to first "hardcode" it as an option to the server executable (which would probably execute a command in the console) and later on create the necessary Lua scripting APIs to change it during runtime and handle it appropriately.