Delete the old truncate_post_name; it was called too late (if url rewriting
is enabled which it is by default), didn't run (it tried to use + to concat
a Fixnum onto a String), and even with those problems fixed, didn't actually
shorten the string enough to use as a pathname.
Instead, apply simple string truncation at the point we generate the slug,
which is used in the filename and is the part that could be unboundedly
long. I arbitrarily chose 200 as the maximum length; even shorter might be
better (really long slugs are just visually ugly); it might also be nicer
to truncate at a hyphen boundary.
This fixes the rest of issue #773 (subissue 4).
fbc9d0c663