Explicitly force 404s on non-WebPath requests

Because the DocumentRoot is set to share/html, a request for /index.html
with WebPath set to /rt still _succeeds_, and presents the Mason source
for share/html/index.html

Add an explicit rewrite rule to 404 anything which is not under the
WebPath.
Merge branch '4.0/devel-apache-404s' into 4.0-trunk