MINOR: proxy: parse mode on dynamic backend creation

MINOR: proxy: parse mode on dynamic backend creation

Add an optional "mode" argument to "add backend" CLI command. This
argument allows to specify if the backend is in TCP or HTTP mode.

By default, it is mandatory, unless the inherited default proxy already
explicitely specifies the mode. To differentiate if TCP mode is implicit
or explicit, a new proxy flag PR_FL_DEF_EXPLICIT_MODE is defined. It is
set for every defaults instances which explicitely defined their mode.