Allow nice_debug_enable and nice_debug_disable to be called before the first NiceAgent is instanciated

Jakub Adam authored Oct 03, 2010 and Youness Alaoui committed Oct 04, 2010

Added comment explaining need of nice_turn_socket_set_ms_... functions
in priv_map_reply_to_relay_request().

Fixed bug in nice_turn_socket_new() where by accident I put both  STUN_AGENT_USAGE_SHORT_TERM_CREDENTIALS
and STUN_AGENT_USAGE_LONG_TERM_CREDENTIALS as usage flags to
stun_agent_init().

The agent->local_addresses should not change when we gather_candidates, let
the user set it with nice_agent_add_local_address, if he does not, then we
should auto-discover interface *during* the gathering process, and not save it.
Currently, when we call gather_candidates, we discover interfaces and save it to
agent->local_addresses as if someone called the _add_local_address API... problem
is that if we have 2/3 interfaces, we do an audio call.. an interface disappears
(vmware closed/whatever) then we upgrade to a video call, then libnice will reuse
the old interface list instead of the current ones..

Conflicts:
	agent/agent.c

wtf.. should not set added to -1 since we continue adding other candidates and e want to schedule_next if one succeeded