Simplify addition of category labels to CPT files

[PaulWessel]

See #4388 for background. This PR enhances the -F+c[label] in makecpt and grd2cpt to take an optional modifier argument to +c that will do the following, depending on label:

1. If label is an integer, say 0, then we auto-add labels 0, 1, 2, ...
2. If label is a letter (say g or G), we auto-add labels g, h, i or G, H, I
3. If any of the above is followed by a hyphen, then we instead set labels as 0-1, 1-2, G-H, g-h, etc.
4. If label contains a comma then we will use the comma-separated items as verbatim labels, until we run out, e.g.,

gmt makecpt -Ccubhelix -T0/3/1 -F+cClouds,Trees,Water
0	24/45.5/72.5	L	;Clouds
1	159.5/121/72.5	L	;Trees
2	194/215.5/242	B	;Water

Skipping a particular label is allowed by not providing one (e.g., Clouds,,Water).

I added a test to show the various cases (test/psscale/catlabels.sh):

Closes #4388.