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().