Add font-selection mechanism to the paper backend. Since we have no way to

Add font-selection mechanism to the paper backend.  Since we have no way to
load font metrics dynamically, we're restricted to the fonts whose metrics
are compiled into Halibut.  Font structures aren't reused when the same
font is specified twice, nor are unused fonts removed from the output.
Finally, the default configuration overflows lines in the manual, but this
would need a change to Halibut's grammar to fix.

Still, what's there works.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@6667 cda61777-01e9-0310-a592-d414129be87e