Update docs to reflect the fact that mod_dll has been replaced by mod_so.

Update docs to reflect the fact that mod_dll has been replaced by mod_so.
In particular, mark mod_dll as obselete, add Windows information and
examples to mod_so documentation, copy info about creating DLLs from
mod_dll.html to mod_so.html (although this needs more work to add info
for Unix users, since most of the description applies to all systems).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@80623 13f79535-47bb-0310-9956-ffa450edef68