gmtwhich: Rename the long names for arguments of the -G option

[seisman]

Description of proposed changes

gmt which -G take four different values:

a - Place files in the appropriate folder under the user directory (this is where GMT normally places downloaded files).
c - Download to the user cache directory.
l - Download to the current directory [Default].
u - Download to the user data directory (i.e., ignoring any subdirectory structure).

Currently, their long names are "user,cache,current,data", but I feel "auto,cache,local,user" makes more sense.

[joa-quim]

You have to change also the test in .../test/gmtwhich