This PR does several things:

1. It adds -F directive g for grayscale. While a couple of GMT modules (e.g., grdimage, grdview) has a -M option (for converting colors to black and white television via the YIQ transformation), we cannot actually make sure CPT output. With -Fg we now get a single grayscale column wether it was originally gray or had to be YIQ converted..
2. While makecpt had the +kkeys modifier, grd2cpt did not. No good reason other than obviously a keyed categorical CPT cannot be used with a grid since the look-up values are always numbers. But users may still want to make a categorical CPT with keys via gd2cpt that can then be used for non-grid plotting. Thus I duplicated the +k stuff from makecpt to grd2cpt.
3. Because they now have identical synopsis and usage I made helper functions that are called by both instead of maintaining duplicate usage messages. The -Farg string is now also a constant in gmt_constants.h
4. Same deal with the docs: Introduced an include file (explain_cpt_output.rst_) to ensure we say the same thing in the two modules. The updated -F docs now look like this: