Copter: add throttle_zero state
Copter: Change all zero throttle checks that should be conservative to use ap.throttle_zero
Copter: throttle zero debounce to separate function

Also initialise throttle_zero  to true on startup
Treat throttle less than zero as zero
Copter: add land_complete to fence disarm check
Copter: use ap.throttle_zero instead of rc_3.control_in in auto_disarm_check
Copter: auto-disarm if land complete regardless of mode
Copter: add mode_allows_arming function
Copter: move all arm check logic into arm_checks
Copter: allow arming in GUIDED only from GCS

Also changed mode_allows_arming function to accept arming_from_gcs param
Also remove AUTOTUNE from arming list
AC_PosControl: add force_descend option to set_alt_target_from_climb_rate
Copter: use force_descend option on auto landings
AC_PosControl: fix to default force_descend param