Address Redmine Bug #2330 when running on FreeBSD

CFEngine had sent a SIGINT to a process that had started up with the same pid as the previous cfengine run. It looks like this issue was documented in redmine as bug 2330 and the solution is to get the process start time of that pid and make sure its the same time as the previous run.

However, code for obtaining that information was only added for Linux, Solaris and AIX and all other unix like platforms fall back to stub functions.

This pull request adds FreeBSD specific versions of the GetProcessState and GetProcessStartTime functions
Ideally I would want all these conditionals to be feature based, i.e. ```if HAVE_SYSCTL_KERN_PROC_PID```. This would auto-enable your patch on all platforms with such functionality, for example Mac OS X and other BSDs.

Anyway I'm just mentioning it for the sake of completion. The Makefile is already dirty as it is, so this is not a must-fix for the pull request.