#720 plotting from an open file descriptor set to 'volatile' is seek() is not available

[dima]

Hi. Gnuplot can plot from an already-open file descriptor with

plot '<&3'

for instance. This file descriptor may be a file (which is seekable) or a pipe (which isn't). This patch attempts a no-op seek, and tells gnuplot the data is volatile if it fails. This makes plot refreshing work with data piped in this way.

[Ethan Merritt]

Good idea.
I've applied this for both 5.0 and 5.1.