RC_Channel: rename 'Manual' to RCPassThru

this should make it clearer what it does
AP_Scheduler: added load_average() function

useful to give a load average in SYS_STATUS mavlink message
APM_Control: set a small D value by default

0.02 may help with some planes that have slow response

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>