Remove include/site.h

I ran into this header while preparing the version number logic for the new version scheme. This header is partly vestigial from imake days, and partly a bunch of defaults that we never let you configure, and partly some defaults that aren't very interesting to configure anyway. Let's clean things up a bit.

This does have some functional changes, namely that Xdmx will report the same vendor string as the other servers, and the -to -fc and -fn options go away. Meh.