mod_http_oauth2: Support HTTP Basic auth on token endpoint This is described in RFC 6749 section 2.3.1 and draft-ietf-oauth-v2-1-07 2.3.1 as the recommended way to transmit the client's credentials. The older spec even calls it the "client password", but the new spec clarifies that this is just another term for the client secret.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.