Remove some instances where a RewriteBase must be specified

Previously, any time you used a relative substitution in 
per-directory/htaccess context, you needed to specify
a RewriteBase. But in case where the context document root 
and context prefix are known via e.g. mod_userdir
or mod_alias, and the substitution is under the context 
document root, we can determine the replacement automatically.

This makes htaccess files or config snippets a bit more
portable.





git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1668870 13f79535-47bb-0310-9956-ffa450edef68