fix `call-process' call.

pass FILE as an argument to gnuplot, rather than as an input.  this
calls gnuplot like
   gnuplot -persist FILE
rather than
   gnuplot -persist < FILE
the error message now contains useful information (file name, line
numbers), which can be parsed by compilation-mode.
factor out gnuplot-quit
factor out gnuplot-handle-exit-status
minor bugfix

message "Running gnuplot..." should come before calling gnuplot, not
after
comments
display the output in `compilation-mode'

this enables useful things like `next-error' and `previous-error'
functions to run gnuplot on region and buffer

unlike `gnuplot-compile', these do not require gnuplot to be visiting a
file.  however, they produce output incompatible with
`compilation-mode'.
update comments and version #

the output from C-c C-c now essentially works with compilation-mode, so
you can use `next-error' and `previous-error' to navigate.

also, I've included commands to run gnuplot on the buffer and region.
these do not require the buffer to be visiting a file (or for the file
to be saved), but they don't produce output compatible with
compilation-mode.