Sets the current process group for the specified PID, 0
for the current
process. Will produce a fatal error if used on a machine that doesn't
implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
it defaults to 0,0
. Note that the BSD 4.2 version of setpgrp does not
accept any arguments, so only setpgrp(0,0) is portable. See also
POSIX::setsid()
.