Fixed SF.net bug #3552839, check_procs: added -k option to ignore kernel threads

This commit fixes sourceforge.net bug #3552839.
It adds a -k|--no-kthreads option to ignore kernel thread processes.
Please note: currently this feature only works for GNU/Linux systems (due to the fact I have no other systems to test/develop on)
Sorry for that, but I'm sure this can be accomplished by somebody else ;-)
NEWS: Mention the new check_procs -k option

Closes #53.