Failsafe: enable low position failsafe also in Manual Position flight mode

[sfuhrer]

Solved Problem

The "low position accuracy" failsafe (by default: RTL) is currently only executed in Mission and Loiter. I find it also useful in Manual Position flight mode, as that mode can also be used to fly out far and depends on a good position estimate to work properly (aka fly straight).

Changelog Entry

For release notes:

Feature: Failsafe: enable low position failsafe also in Manual Position flight mode