Make URI parser able to tolerate nonconformant URIs.

Make URI parser able to tolerate nonconformant URIs.

If the EVHTTP_URI_NONCONFORMANT flag is passed in (which it is when
parsing URIs we get over the wire), then we relax our checks a lot.
Specifically, we do nothing to check for correct characters in the
path, query, and fragment parts of such a URI.

We could do much more here: we could relax our hostname requirements,
deal with spaces differently/better, trap some errors but not others,
etc.  But this should solve the worst user-agent compatibility issues
for now; the other issues can wait for a later release.