AC_PosControl: bug fix dt calculation

fixes issue in which now could be earlier than _last_update_xy_ms
leading to a large dt value and a sudden lean on takeoff
AHRS_DCM: sanity check AHRS_RP_P and AHRS_YAW_P
AC_WPNav: bug fix sanity check of set_speed_xy

This corrects a bug that allowed the waypoint speed to be set to zero
Copter: print frame type in log headers
Copter: landing detector checks baro climb rate

Barometer climb rate must be -150cm/s ~ +150cm/s
This threshold is generous because we already use the inertial
navigation climb rate so this is just to catch cases where inertial nav
is very incorrect in it's climbrate estimates
DataFlash: log baro climbrate