Make ast_channel_walk become ast_channel_walk_locked

apps/app_agi.c

apps/app_groupcount.c

apps/app_setcdruserfield.c

apps/app_softhangup.c

apps/app_zapscan.c

channel.c

channels/chan_mgcp.c

cli.c

include/asterisk/channel.h

manager.c

pbx.c

res/res_monitor.c

res/res_parking.c