UseD: Fix add_ddoc's handling of excluded sources
UseD: Move remaining bits to COMPILE_OPTIONS
UseD: Document old bug in D property enforcement
UseD: Remove add_ddoc's ability to choose ext

Ddoc extension will now always be html. I was (partly)
mistaken about Ddoc's ability to generate non-html
output, and now understand it that while allowing other
output types was only discussed on the list, the idea
fell apart pretty quickly since Ddoc allows HTML tags
to be manually specified.

Also, LDC refuses any extension other than html, silently
replacing the extension you set.

Conversion from HTML to other formats is still possible
as an additional (user implemented) step, of course.