Document new config option

[dmorissette]

Frank added a new CONFIG mechanism to the mapfile, and this still needs to be
documented.  Here is the note sent to mapserver-dev by Frank on 2004-03-13:

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Folks,

I have added the CONFIG keyword for use in the MAP object in a .map file.
It takes two arguments, a key and a value.  The PROJ_LIB key will attempt to
force PROJ.4 to use the provided value for looking up files like the epsg
file, and the datum shift files.

I am hoping that setting the PROJ_LIB variable will be the main use of
this keyword for most people, helping them avoid having to set external
environment variables.

Any other value will be passed on to CPLSetConfigOption().  This provides
customized control of some GDAL and OGR driver behaviours.   Details on
such options would be found in specific GDAL driver documentation.  My main
reason for implementing all this is because it is needed for controlling some
behaviour of the OGR DODS driver.

In addition to adding the CONFIG keyword in map file parsing, I also add
a setConfigOption() and getConfigOption() method in mapscript.i, and documented
them in mapscript.txt.  I even tested them briefly in a python mapscript script.
I am getting the hang of MapScript!

I haven't updated the mapfile reference yet, but I intend to do that soon.

Changes all committed.  Whack me if I screwed something up.

Best regards,

[jmckenna@…]

Frank, I added this info into the mapfile-reference for the main doc CVS. 
Please check and verify that it is sufficient.  
I also tested it and it works great!  (for setting the PROJ_LIB epsg location
that is)