Copter: flag stays landed unless pilot raises throttle
INav: clear historic z-axis position estimate when set_altitude called
Copter: set inav alt to zero when arming
AP_Math: fast_atan2

This is 126us per call vs 199us on the AVR.  it is accurate to about
0.28 degrees

Committed by rmackay9 but contribution is from David Dewey
CPUInfo: add fast_atan2
Copter: use fast_atan2 for bearings to home and next WP
AC_WPNav: use fast_atan2 for bearing calcs
AC_Circle: use fast_atan2 to calc bearing from center

This does not save much time because it's only called at initialisation