Improve documentation of the mapfile security things

[jratike80]

There should be one locasion in some easy place in the documentation that collects all the security maneuvers (magic strings, expression validation) which are compulsory or useful in the mapfile. I think that a new subtitle "security" under mapfile document http://mapserver.org/mapfile/index.html might be a good place. To begin with, the security section might contain the main points of ​http://lists.osgeo.org/pipermail/mapserver-users/2009-March/060600.html and some information about where and how validation rules are used. Now this essential information is scattered around the documentation.

[havatv]

Should include the Apache variables introduced in RFC56 (MS_MAP_PATTERN and MS_MAP_NO_PATH). See also #2977. These don't seem to have a home at the moment.