Copter: added FS_THR_ENABLED_ALWAYS_LAND (=3) option: don't attempt RTL,...

Added a 3rd failsafe option, ALWAYS_LAND for cases in which attempting RTL may not be the best thing to do when something goes wrong. 
e.g. when flying in small / enclosed areas or where GPS drift may be a concern.

- Rob

**Updated to upstream/master