Merge disorder.macros branch.

Merge disorder.macros branch.

This is a major rewrite of the web interface.  The template language
has been changed and is hopefuly easier to use.  Much of the
implementation has moved to lib/, along with some of the CGI support.
The CGI can now figure out its own URL, including HTTPS URLs.

The web interface documentation is no longer mixed into
disorder_config(5).  The top level is disorder.cgi(8) but there are
several related pages, much of the content generated from source code
comments.

The server now unsets track preferences if you try to set them to
their default value.  This resolves a long-standing TODO.  The server
is otherwise largely unchanged.

This changes fixes defects 2, 12 and 18 (the first and last of these
being the payoff for casual users).