This command was not implemented fully removing this code resolves a
compiler warning

This avoids the chance of an arithmetic exception in cases where the
position is moved too far

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.

Split of NAV_GUIDED into these two command necessitated guided mode
store the limits

Thanks to roque-canales for raising the issue and paradisephil for finding
the specific piece of code that went wrong and suggesting the fix.

Moved ROI logic to new set_auto_yaw_roi function.

pair programmed with Craig Elder

Fix #1068. When receiving guided waypoints do not change to GUIDED mode. This serves as a safety precaution for GCS, since they must switch to guided mode before sending the waypoints.

Due to a race condition, the wp_distance was not being updated before
the conditional_distance

waypoint command was setting auto_yaw_mode when it was run after the
do-cmd
yaw_look_at_heading was being set to current heading which was
overwriting the caller's desired heading

Next waypoint's location must be passed in even if it's a straight
segment.
mission.get_next_nav_cmd's start_index should be the current command +1

To-Do: extend wpnav yaw control to only repoint nose towards waypoint if
it is more than 5m or 10m away

It is unlikely that the "f" at the end of the 100 is required to
maintain the full precision of the target speed but added just in case