This allows us to remove a bunch of tangled #ifdefs and some local
PPS API headers that are almost certainly obsolete.

Affects old Sun, Solaris, and SCO systems. Touches the parse driver
and the MX4200 refclock (the latter only trivially).  Leaves the
RFC2783 support itself in place.

Leaves the parse driver documentation possibly a bit more out of sync
with reality than it was, but the reader has already been warned that
it is stale.

Research suggests that the last system for which any of this stuff was
relevant shipped in 1996. Compilation should break noisily if anyone tries
to build on a system old enough for it to matter.