autotest: fixed sleep_overhead calculation in sim_multicopter.py
AC_WPNAV: check distance to waypoint within library

bug fix for loiter using lat/lon position instead of NED position when
calculating desired velocity towards target
Copter: move alt and wp checking to AC_WPNAV

RTL fix so that if it starts rtl-ing from above 80m it returns home
while descending instead of descending at initial position.
add get and set_target_alt_for_reporting
AC_WPNav: fix compiler warning