Copter: ensure lat, lon, alt all zero is handled

Use default position when waypoint lat/lon are zero,
and default altitude when waypoint alt is zero, for
both spline and straight-line waypoints.

Fixes #1145.
AutoTest: copter test that zero alt defaults to current alt
AC_WPNav: minor fix to comments
Copter: clarify pv_location_to_vector param name

Also update function description