output/gnuplot: Use .dat as suggested file extension.

output/gnuplot: Use .dat as suggested file extension.

Gnuplot doesn't have any "official" file name extension(s). It uses
(at least) two different types of files basically:

 - "control files": These can have many different somewhat commonly
   used extensions such as .gpi, .gnu, .gnuplot, .gp, .plt, .gih,
   others. These files don't contain data, only Gnuplot commands such
   as 'set yrange [75:105]', 'set ylabel "foo" offset 1', and so on.

 - "data files": This is what libsigrok reads and writes. These files
   contain actual data to be graphed by Gnuplot (with the help of a
   specially-crafted control file, see above). The data is usually in
   a tab-separated format. The common file extension is usually .dat,
   though many others are possible as well.