cmd: extend 'cfg' to include computed graph metadata.

notably loop depth / header information.  also added an optional
argument to the command so that the graph can be output to a file
instead of stdout.
cfg: fix depth calculation bug.

in a nested loop situation, the "fail" conditional on a nested
loop was being considered as "under" *both* loop headers, when in
reality it is only technically part of the outer loop.  this uses
a slight modification to the reachability analysis to make sure
only the "relevant" loop headers actually have an effect.
cmd, cfg: try to get graphs a bit wider.

tall graphs suck for inserting into e.g. latex.