Make mod_include use a hash table to associate directive tags with

Make mod_include use a hash table to associate directive tags with
functions.  This allows modules to implement their own SSI tags easily.
The idea is simple enough, a module can insert it's own tag and function
combination into a hash table provided by mod_include.  While mod_include
parses an SSI file, when it encounters a tag in the file, it does a
hash lookup to find the function that implements that tag, and passes
all of the relevant data to the function.  That function is then
responsible for processing the tag and handing the remaining data back
to mod_include for further processing.
Submitted by:	Paul J. Reder <rederpj@raleigh.ibm.com>
Reviewed by:	Ryan Bloom

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87241 13f79535-47bb-0310-9956-ffa450edef68