Add support for parking with IAX2

channel.c

channels/chan_agent.c

channels/chan_alsa.c

channels/chan_h323.c

channels/chan_iax.c

channels/chan_iax2.c

channels/chan_local.c

channels/chan_mgcp.c

channels/chan_oss.c

channels/chan_sip.c

channels/chan_skinny.c

channels/chan_vpb.c

channels/chan_zap.c

include/asterisk/channel.h

include/asterisk/channel_pvt.h