[PATCH, RFC]: Get rid of NEED_POSIX_SETPGID & SETPGRP_ARGS

[Mark Kettenis <kettenis at science dot uva dot nl>]

Here is a patch that gets rid of two host-specific macros and replaces
them with autoconf checks:

 * I don't think we need NEED_POSIX_SETPGID.  I think we should always
   prefer setpgid over setpgrp.  There is no point for GDB to become a
   session leader (which setpgrp () might try to accomplish), we just
   want to have it in its own process group.

 * SETPGRP_VOID is what autoconf calls the opposite of SETPGRP_ARGS.

inflow.c has no maintainer, so if I don't hear any objections before
saturday july 21st, I'll check this in and remove all traces of
NEED_POSIX_SETPGID and SETPGRP_ARGS.

Mark