output: Make default output file name depend on the format.

output: Make default output file name depend on the format.

Before this commit, "-O html" caused HTML output to be written to a file
named "-", which is confusing.  After this commit, it is written to a file
named "pspp.html".  Similarly for other output drivers, except that text
and CSV output are still written to stdout by default.

Bug #35030.
Reported by John Darrington.