libcupsfilters: In cupsRasterPrepareHeader() added monchrome PCLm support

libcupsfilters: In cupsRasterPrepareHeader() added monchrome PCLm support

PCLm supports exactly 2 color spaces, sRGB 8-bit for color and sGray
8-bit for monochrome printing. Up to now we only had supported the
sRGB space. This commit adds support for sGray.

This especially allows monchrome printing on color PCLm printers, and
faster (less data to handle) and more economic (use only black
toner/ink, not the colored ones) grayscale printing.

Principally all PCLm printers should support both color spaces.