Returns the current process group for the specified PID. Use a PID of
0 to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement getpgrp(2). If PID is omitted, returns the process group of the current process. Note that the POSIX version of
getpgrp does not accept a PID argument, so only
PID==0 is truly portable.
Portability issues: "getpgrp" in perlport.