Stupid bug if filename begin with whitespace

[Ethan M. <merritt@u.washington.edu>]

Stupid bugs in set.c:

gnuplot> set output "   outfile"
gnuplot> set output "   die"
Segmentation fault (core dumped)

and similarly for 'set print'.
The code was very sloppy; it allocated a string to hold the filename,
and then advanced the string pointer over the initial whitespace.
When the string is freed, boom!

I have removed this "feature" from the two places that used it.
Is it important enough to remove whitespace that we should have a
separate, more correct, routine that does so and share it for all
places that a filename is read in?