Improved mime-type guessing (issue #1233)

While performing a random walk through the issue tracker, I came across issue
#1233, which includes a patch to use the /etc/mime.types file on *nix systems to
improve our mime-type guessing. It looked interesting, so I started digging in,
only to realize that there were more issues at play than at first blush.

Before go too deeply into the code, I have a few questions I was hoping people
could answer:

  * Does this even matter? Would adding this functionality be a benefit to our
    users, and if so, does the overhead of adding and maintaining it outweigh the
    derived benefit?

  * Is there a better place to do this? APR? Would it be reasonable to move the
    mime-type handling code out of httpd into APR where Subversion could make use
    of it?

  * Is there equivalent MIME type guessing functionality available on Windows,
    either through an API or via a system file?

Hopefully the answers to these questions will help us resurrect this patch, or
decide on a reasonable fate for issue #1233.

-Hyrum