Update rbtrace.c

Added definitions for PLATFORM (PLATFORM_FREEBSD, PLATFORM_LINUX) on lines 44-50.

Added setpgid(0,0); command for FreeBSD which should also be fine using Linux (except CentOS I am not sure) on lines 967 to 972. In this case I left the old command setpgrp(); for linux just in case. But in theory using 'setpgid(0,0);' should work on all platforms according to all documentation I've run into.

Anyway all the documentation for FreeBSD and Linux & other say:

 The call setpgrp() is equivalent to setpgid(0,0).

Please do check and Test. :-)
Merge pull request #24 from kiwicustom/patch-1

Update rbtrace.c