plot: add gnuplot 5 support

plot: add gnuplot 5 support

gnuplot 5 no longer supports using old-style ($1, $2 etc.) gnuplot 4 parameter
substituion causing fio2gnuplot to break on newer Linux distributions.

Fix this by using new-style arg variables everywhere and conditionally convert
old-style substituions to new-style variables. Add some casting to the average
variable so it is treated as a number or string appropriately and bail out with
an error message when too few parameters are passed in.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>