rgw: log civetweb messages

Handle the civetweb log_message callback, divert messages into our debug
log.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: don't allow negative / invalid content length

Certain frontends (e.g., civetweb) don't filter such requests.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: update civetweb submodule

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>