fio2gnuplot: Adding comparing 2D graphs

It's a pretty common need to plot several files on the same graph to
compare behavior during various benchs.

If 2 or more files are selected by the pattern, new plots will be
generated to compare the traces into a single graph. 3 kinds of graphs
will be generated (raw, smooth and trends) like for the individual
graphs.

The output filename will be prefixed with "compare-" like :
compare-vm1-1-4m-vdb-read-para.results_bw-2Draw.png
compare-vm1-1-4m-vdb-read-para.results_bw-2Dsmooth.png
compare-vm1-1-4m-vdb-read-para.results_bw-2Dtrend.png
fio2gnuplot: Fixing help message

Some options were missing on the main description.