AP_GPS: added GPS_SBAS_MODE parameter

allows SBAS to be enabled/disabled
AP_GPS: added GPS_MIN_ELEV parameter

allows setting of satellite elevation mask in degrees
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.