Introduces mod_mime AddInputFilter and AddOutputFilter filter ext syntax.

  Significantly refactored the code to maintain single functions for the
  add_extension_info hash and remove_extension_info table processing.

  The AddInputFilter/AddOutputFilter could have been plural, as they accept
  a semicolon delimited list of filters (e.g. you need to stack two different
  filters based on a single extension.)  I expect that the plural could
  confuse users, though, as they might expect AddInputFilters foo bar var
  to add foo and bar to the extension var, which it definately doesn't do!


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90756 13f79535-47bb-0310-9956-ffa450edef68