Minor Nav code improvements

[breadoven]

PR makes following changes:

1. Improves WP reached detection for fixed wing to remove dependency on nav_fw_loiter_radius. Detection now relies on checking bearing to WP relative to initial bearing to WP which is more reliable than checking distance based on a loiter radius that may not actually be achieved if set incorrectly.
2. Adds new Nav state flag NAV_CTL_HOLD to define when Nav hold states are active or will become active when WP is reached. Cleans up check for Nav Hold being active.
3. Replaces "Hover" with "Loiter" where relevant which seems more appropriate given planes generally can't hover.
4. Moves Landing Detected arm blocker reset code to fc_core.c updateArmingStatus which is the more correct location than in the landing detection code itself.

Tested OK for fixed wing on HITL.

[Jetrell]

It works great.. Thanks.
I ran it through some back to back tests against the current method.. And even set nav_fw_loiter_radius to a low 5m. Which it couldn't possibly do. But it still made the safehome location each time based on the bearing, and instantly start descending to the nav_rth_home_altitude or Landing Approach altitude and loitered around.

[breadoven]

Yes, using using relative bearing is a much better method than distance for planes that loiter around and never actually reach the waypoint itself. Need to check this with multirotor though just to be sure odd things don't happen, might be possible in the next few days.

[breadoven]

Managed to flight test multirotor with these changes and everything worked as expected for WP and RTH modes so this should be good to merge.