Add socket activation for systemd, with a option to stop on idle

Systemd support socket passing on activation. While the goal is to make sure every can be started in almost any order, it also permit to have on demand start. 

This series of patches implement systemd support using the library provided by systemd ( and proper autodetection on build time, so someone not wanting the code can just remove systemd include, like that's done for selinux ), and add a option to shut down the service on idle. The idea is that if the service is not used, the memory can be freed and the service will be restarted after if needed. I also added 2 examples units.

( and yes, i found out after http://dev.kryo.se/iodine/ticket/106 , but I still propose my patch since it reuse a library instead of copying the code, and is more modular )