Improved synopsis and usage of -Ccpt for grid plot modules

[PaulWessel]

The grid plotters grd2kml, grdimage, grdview, and grdvector may take a bit more complicated CPT argument. This is now documented better via API function gmt_explain_cpt_input and RST include file use_cpt_grd.rst_. Also standardised these four's synopsis syntax via a new constant CPT_OPT_ARGS.
While making C changes this is all about documentation only. For instance, the grd2kml usage message looks like this:

-C[<section>/]<master>|<cpt>|<*color1,color2[,color3,...][+h[<hinge>]][+i<dz>][+u|U<unit>][+s<fname>]
     Color palette file to convert grid values to colors, provided by one of three ways:
       • Name one of the master CPTs (no extension) [turbo].
       • Name a local regular CPTs (with extension).
       • Specify <color1>,<color2>[,<color3>,...] to build a linear continuous CPT to automatically assign continuous colors over the grid data range.
     A few modifiers control generation of the active CPT:
       +h Enable a soft hinge in your master CPT. Append <hinge> to change its value.
       +i Append <dz> to quantize the grid range [Default uses the exact grid range].
       +u Scale CPT z-values from another <unit> to meters.
       +U Scale CPT z-values from meters to another <unit>.
       +s Append <fname> to save the generated CPT to file.
     Note: For converting units with +u|U, see units e|f|k|M|n|u.

and the man page looks like this now: