change td_retval into a union w/ off_t, with defines to mask the
change...  This eliminates a cast, and also forces td_retval
(often 2 32-bit registers) to be aligned so that off_t's can be
stored there on arches with strict alignment requirements like
armeb (AVILA)...  On i386, this doesn't change alignment, and on
amd64 it doesn't either, as register_t is already 64bits...

This will also prevent future breakage due to people adding additional
fields to the struct...

This gets AVILA booting a bit farther...

Reviewed by:	bde
copy these files from lib/libmd in preperation for moving these files
into the kernel...
Xref pthread_cancel...

bump Dd, _exit was still dated 1996!  yes, it has been modified a few
times since then...

MFC after:	1 week