I tried Andreas's suggestion of disabling ISIG, but
this didn't work on Solaris, and it doesn't look like
it'd work on any host where USABLE_SIGIO is false.

Instead, how about the attached patch?  It reenables
the BSD_PGRPS idea, although reimplemented, as the old
code can't easily be resurrected as-is.  I haven't
thought about how this affects the Windows port, so
I'll CC: this to Eli.