fio2gnuplot: Option to select gnuplot's output dir

This patch add the '-d' option to select where gnuplot shall render its
files.

When plotting big series of files, it's pretty anoying having all the
files in the current directory as there is many many of them.

So let's use this option to allow users specifying where all the
graphics shall be rendered
Makefile: Installing fio2gnuplot & sons

Let's install fio2gnuplot & other gnuplot files (.gpm) while installing
fio.

Gnuplot files are installed into $(prefix}/share/fio/ subdirectory
Makefile: Installing genfio at install time

genfio wasn't installed at all during 'make install'.
Tree: Moving fio_generate_plots in tools/

As fio2gnplot.py & genfio are located in tools, it sounds better to put
all the additional tools in the same directory.
fio2gnuplot: Using default install dir for gpm files

That was pretty boring trying to read the gpm files from the same dir as
the logs. Since gpm files are now installed in a specific dir at make
install time, let's use them from here.

Two locations will be considered at runtime : /usr/share/fio and
/usr/local/share/fio
fio2gnuplot: Printing number of selected files

when using pattern matching on a big set of files, it's pretty hard to
estimate if we missed some or not.

Let's print the number of matched items with the selected pattern to
help user estimate if it's ok or not.
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.
fio2gnuplot: Adding information steps

While executing fio2gnuplot, we were lacking of some information of
what's going on.

This patch makes things easier to understand/follow for end users.
fio2gnuplot: Adding filename as title for 2D graphs

While plotting 2D graphs, it's pretty handy to keep the filename as
title for the plotting trace. This way, the png file still keep the
original filename used for plotting.