Improve the debug message about the default projection

[seisman]

When running a command like gmt coast -RJP -W1p -pdf map -Vd, the debug message says:

gmt [DEBUG]: Modern: Adding -JQ15c+ to options since there is no history available.
gmt [DEBUG]: Revised options: -RJP -W1p -Vd -JQ15c+
coast [DEBUG]: History: Process -RJP
coast [DEBUG]: History: Process -JQ15c+

-JQ15c+ is the old syntax so it's a little misleading. The new syntax should be -JQ15c+du (or maybe -JQ15c+dw).