lpr: replace setpgrp(2) with setpgid(2).

setpgid(2) is more portable than setpgrp(2).
The BSD variant of setpgrp is a wrapper for
setpgid(2) anyways.

MFC after:	5 weeks
unistd: move setpgrp(2) to the __BSD_VISIBLE section

Our setpgrp(2) differs from the specified by POSIX, which
only has one argument, and is only meant for compatibility
with BSD.

Reference:
http://pubs.opengroup.org/onlinepubs/009695399/functions/setpgrp.html

Pointed-out in:	openbsd-tech (2014-09-16)
MFC after:	6 weeks