Add net-proxy/haproxy, modified from official ebuild.

Modifications:
- Add patch to trigger health check immediately after a reload for all
  backends. Without this, immediate check only happens to the first backend
  defined. See mailing list threads:
  - http://thread.gmane.org/gmane.comp.web.haproxy/14644
  - http://thread.gmane.org/gmane.comp.web.haproxy/14789