Use wsgi.url_scheme instead of ad-hoc CGI checks.
Instead of each place in hgweb_mod that needs to check for SSL or get
the protocol scheme (http vs. https) doing it ad-hoc, make them just
look at the wsgi.url_scheme which because of previous patches is now
always set correctly.
Make hg serve set the wsgi.url_scheme property correctly.
This fixes the bug where hg serve with SSL will always detect the wrong
urlbase for templates. This is also part of unifying the CGI interface
and hg serve interface to both use standard wsgi.