Copter: add short delay to arming to allow RC input

The short delay gives time for the RC inputs to be processed which
removes the chance of a false-positive on the "late frame" radio check.
A false positive could lead to an immediate disarm right after arming.
Copter: land check gets overall throttle and rotation rate check

add check that overall throttle level is below the non-takeoff throttle
instead of just checking that it's motors have hit their lower limits
because low limits can also be caused by high yaw rotation requests.
Absolute climb rate requirement reduced to 30cm/s
Copter: pre-arm consistency check of gyros