CH 7/8 OPT Retract Mount

-Define AUX_SWITCH_RETRACT_MOUNT in defines.h
-Added 25=Retract Mount to Parameters.pde CH7 and CH8
-added int on CH7/8opt AUX_SWITCH_RETRACT_MOUNT
-Added Case #if Mount == ENABLE to set mount mode
made set_mode public in AP_Mount.h lib
Mount mode is set to saved value when channel goes low

Tested on the bench with APM 2.5
-Mission Planner needs to get this option inn also