Add docs for map=<anv_variable_name> in mapserv CGI URLs

[dmorissette]

There is an undocumented map=<env_variable_name> mechanism that can be used to refer to mapfiles via an environment variable instead of exposing full mapfile paths to the outside world.

This needs to be documented in the CGI docs at ​http://www.mapserver.org/cgi/controls.html

Example, set MY_MAPFILE in Apache's httpd.conf:

  SetEnv MY_MAPFILE "/opt/mapserver/map1/mymapfile.map"

... and then refer to it as follows in the mapserv CGI URL:

  http://localhost/cgi-bin/mapserv?map=MY_MAPFILE&mode=...

[havatv]

Fixed for 6.0 and trunk in r12010.