Description of the desired feature

Originally posted by @PaulWessel in #6190 (comment), this feature request is for MAP_LENGTH_UNIT as a way to change the default unit for map distances. To summarize the discussion in that thread, the default is not currently consistent with most places having m but several places (-SE and likely map project, grdproject, and project) having km as the default.

Example usage:

echo "0 0 20" | gmt plot -SE- -Bafg10 -JH10c -Rd -W2p,green --MAP_LENGTH_UNIT=d -png test

for the same results as:

echo "0 0 20d" | gmt plot -SE- -Bafg10 -JH10c -Rd -W2p,green -png test