The howpublished field does not show in Beamer slides for instance.
It seems that "howpublished" is intended for "@misc" only, but it's
use here is pervasive.  I believe it is saner, and yields better
results, to use "note".

More bib files should be converted.

It would be a good idea to automate this conversion, but also to
check that these URLs are still alive.

* bib/comp.lang.c++.bib, bib/compilation.bib: Use "note" for URLs,
not "howpublished", "annote", or "url".