BUG/MINOR: config: Fix parsing of accept-invalid-http-{request,response}

BUG/MINOR: config: Fix parsing of accept-invalid-http-{request,response}

These options are now deprectated, but the proxy capabilities are not
properly checked during the configuration parsing leading to always ignore
these options. This is now fixed by checking the frontend capability for
"accept-invalid-http-request" option and the backend capability for
"accept-invalid-http-response" option.

In addition, the messages about the deprecation of these options are now
emitted with ha_warning() instead of ha_alert() because they are only
warnings and not errors.

This patch should fix the issue #2806. It must be backported to 3.1.