Improve "make install" to help with packaging

Various changes to make coprthr easier to package and distribute, mostly related to implementing the DESTDIR variable. See www.gnu.org/prep/standards/html_node/DESTDIR.html for information about DESTDIR (Arch Linux frequently uses it when building packages).

The use of sysconfdir and localstatedir as opposed to "enable-user-install" should also make it easier to prepare a FreeBSD port--from what I recall of the ports tree.