update information about debugging sysctl...

MFC after:	1 week
Backport upstream r684 (OPENPAM_DEBUG enables debugging macros but does
not turn debugging on by default) and add OPENPAM_DEBUG to CFLAGS.
Add fdallocn function and use it when passing fds over unix socket.

This gets rid of "unp_externalize fdalloc failed" panic.

Reviewed by:	pjd
MFC after:	1 week