fix navigator bug: "Takeoff to x meters above home." after mission ended

[bkueng]

Fixes #14011

The problem described there actually goes deeper: the message is also sent when switching to mission mode while disarmed and with a valid mission. Even worse: if the first mission item is a command, it will be published at the wrong time.

The root cause is that the (mission) state machine is executed too early/at the wrong time. This is fixed with this PR by going into navigator idle mode while disarmed (i.e. not entering mission until armed).
All existing arming checks in navigator are unaffected by this, except that there is a slight risk that if takeoff is detected immediately upon arming, the first takeoff waypoint is converted into a position waypoint (however the position controller has its own takeoff logic anyway).

Another consequence is that mc_pos_control starts to complain about invalid setpoint triplets while disarmed. The PR resolves this by also accepting invalid triplets and interpret them as loiter.
The risk with this is that it could hide problems, where navigator sends invalid triplets when it should not.

Best reviewed by commits.

This has been quite extensively SITL + flight tested, MC + VTOL. Thanks to @julianoes and @sfuhrer for the support.

Log: https://logs.px4.io/plot_app?log=66a6062f-cbdb-47cd-83f3-fc71724b58c5

[dusan19]

FYI:
With this pr, in auto modes the triplets are published on every navigator iteration when disarmed.
This condition passes in navigator_main.cpp and sets the triplets updated flag:

		/* if nothing is running, set position setpoint triplet invalid once */
		if (_navigation_mode == nullptr && !_pos_sp_triplet_published_invalid_once) {
			_pos_sp_triplet_published_invalid_once = true;
			PX4_INFO("nothing running reset triplets once");
			reset_triplets();
		}

Before this pr, the triplets were reset and published only once when the drone is disarmed and in nav state AUTO LOITER for example. Now they are published all the time (disarm the drone and set it in auto loiter to reproduce)
It doesnt really effect anything, just wanted to point out the different behavior.

[julianoes]

@dusan19 I recently realized that too, thanks.