Path saving: erase previous waypoints before saving

This erases all consecutive (found) waypoints in flash for
the specified path.  This is necessary if there are more
waypoints in the flash chip than ram, which would result
in only some of them being overridden.
Path loading: mark any waypoints in ram not used invalid

Because we cannot unallocate waypoints, if previously a path
had more waypoints than the one being loaded, those waypoints
will still exist in ram.  Any remaining ones are set to
INVALID to indicate they should not be used.