r.mapcalc: add -l flag

[martinl]

Add new flag to list inputs and outputs:

r.mapcalc -l "test1 = test2 + test3 + 5"
input=test2,test3
output=test1

From #2409:

 Such an option would be fairly simple to add. Most of the relevant logic is already present in execute() in r.mapcalc/evaluate.c.

The initialize() function populates the list of input maps (in r.mapcalc/map.c or map3.c), which can be used to generate the list of inputs. Each top-level assignment expression defines an output.