Revision 413035

- update to 2.4.20
- use ${OPTION}_IMPLIES and remove some IGNORES
- turn on proxy_html and xml2enc as default [1]

[1] Often requested by users. The modules are not enabled in
    the default configuration.

Full changelog for apache version 2.4.19/2.4.20:
 http://www.apache.org/dist/httpd/CHANGES_2.4.20

Note: Apache httpd 2.4.19 was not released.

MFH:	2016Q2