fio2gnuplot: Adding manpage

As requested by users, let's add a manpage to make things easier to
understand.

I'm providing here the raw file used to generate the manpage. It can be
converted to a real manpage by using txt2man.

I'm also providing the manpage file to avoid dependencies at build time,
anyway the makefile target can be easily used to rebuild it if
necessary.
genfio: Don't consider only /dev/ disk device

Prior this patch, genfio was considering the disks __NOT__ having the "/dev"
name in it.

That's a serious limitation as
- we can't test regular files
- that doesn't works for complex /dev/ naming like /dev/disk/by-id/...

This patch removes this requirement and let user giving the complete
path.