Place mandoc.css into the public domain.
The reason for doing this rather than using the ISC license
is that i guess that in some contexts, a requirement to preserve
a Copyright and license header might be inconvenient, and i really
don't care at all how people use it.
What matters is that they do use it, or something similar - attempts
to use mandoc without any CSS are a constant source of grief and
bogus bug reports because HTML without CSS doesn't look very good:
the more structural and semantic and the less presentational and
old-fashioned the HTML, the more so.
Thanks to Mark Harris <mark dot hsj at gmail dot com> for pointing out
that the permissions on this particular file were unclear.