Allow a hastily selected subset of the output formats to also accept

"-" as a special file name meaning standard output. I've restricted
it to just those output formats which can predictably output only
one file, just for the sake of not having to faff too much with the
others.

Probably what I should have done for all of this would have been to
write a set of wrappers around fopen, fclose and everything in
between, and use them everywhere. Those wrappers would uniformly
detect "-" and convert it into stdin or stdout as appropriate, would
avoid fclosing those files for real when told to close them, and
would also be able to handle reading a little bit of data from the
start of a file and then pushing it all back even if the file were
not seekable (which would allow input.c to lose the ugly special
case whereby it can't currently read font files from standard
input).

git-svn-id: svn://svn.tartarus.org/sgt/halibut@8729 cda61777-01e9-0310-a592-d414129be87e