Copter: reduce alt hold defaults

Throttle Rate P to 5.0 (was 6.0)
Throttle Accel P to 0.5 (was 0.75)
Throttle Accel I to 1.0 (was 1.5)
TradHeli: update AttControl params to match multicopters
DataFlash: allow use of a smaller writebuf for PX4v1

this fixes logging on PX4v1