fio2gnuplot: Adding time selection options

It's could be very handy being able to extract a portion of a trace to
plot it.

This patch adds two new options to select the starting time and the end
time were the logs shall be considered for plotting.
fio2gnplot: Renaming file variable to myfile

'file' could be reserved on some modules or in the python.
It's better to rename it to avoid troubles.
fio2gnuplot: Fixing plotting issues on shorten files

fio2gnuplot was mixing files when some got shorten.

The position inside the array was used to remember what file was read
leading to mistakes once some values got ignored.

As a fix, we shall count the index for every single line we read and add
to the time & perf we keep. While writing data to the temporary files,
the extraction of the index insure we write to the proper file.

Before this patch, we observed at the end of the traces some stupid artefacts like loops... was
looking like a 4years old kid drawing...

That's now fixed.