[MEDIUM] Implement and use generic findproxy and relax duplicated proxy check

This patch:
 - adds proxy_mode_str() similar to proxy_type_str()
 - adds a generic findproxy function used with default_backend/setbe/use_backed
 - rewrite default_backend/senbe/use_backed to use introduced findproxy()
 - relaxes duplicated proxy check
 - changes capabilities displaying from "%X" to "%s" with a call to proxy_type_str()
3 files changed