Add support for 2-band grayscale w/alpha rasters

[chodgson@…]

Get mapserver to automatically recognize and use alpha channels for transparency
in grayscale images - 2-band tiffs, JPEG-2000 with alpha band, etc. Chris
Hodgson will GREATLY appreciate this feature :)

[fwarmerdam]

Support implemented for greyscale, pseudocolored and classified (8bit) image
files with an alpha band.  This can be forced by using BANDS=1,0,0,2 or if
the file itself has the second band properly marked as alpha it should
get used automatically. 

The alpha values are just used as an on-off flag (1-127=off, 128-255=opaque)
if the output is 8bit, or as proper alpha if the output is truecolor. 

Added the test msautotest/gdal/greyalpha.map in the test suite to verify 
correct operation. 

[chodgson@…]

I've tested this a mix of greyscale/alpha and grayscale/no-alpha jpeg-2000
images in a tileindexed layer without a "BANDS" directive, it works great.

Not sure if it's my job to mark this as verified or not but I will...