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