lib: http: poison parser references after freeing

Make it a little harder to slip in use-after-free bugs by nulling out
references to the parser object after handing it off to freeParser().
lib: unexport http.parsers

Unexport the http.parsers freelist.  It was originally exported by Ryan
in commit 0003c701 but the commit log doesn't mention why and it's never
been documented.  It's unclear if there are any users.

The lifecycle of parser objects changed recently and it seems better to
not let people shoot themselves in the foot so easily.

If it turns out there are actually users, we can always re-export it
again - probably under a slightly different name, to force people to
update their code to the new way of things.