[patch] check_instance() isn't working on non-GNU ps

Good day!

It's still possible to start more then one gtg instance in FreeBSD (and i believe in other BSDs), because the check_instance() in GTG\gtg.py is GNU ps(1)-specific. I have the following message when starting gtg on FreeBSD:

> gtg
ps: illegal option -- -
usage: ps [-aCcdefHhjlmrSTuvwXxZ] [-O fmt | -o fmt] [-G gid[,gid...]]
          [-M core] [-N system]
          [-p pid[,pid...]] [-t tty[,tty...]] [-U user[,user...]]
       ps [-L]
ps: illegal option -- -
usage: ps [-aCcdefHhjlmrSTuvwXxZ] [-O fmt | -o fmt] [-G gid[,gid...]]
          [-M core] [-N system]
          [-p pid[,pid...]] [-t tty[,tty...]] [-U user[,user...]]
       ps [-L]

I've attached ugly but simple patch, that works fine both on linux and freebsd.