AP_Mission: Change jump_to_landing_sequence to get_landing_sequence_start
Plane: Bugfix.  Changing to landing sequence now works at mission end.
Plane: minor code style change

use comment and function style consistent with rest of code
Plane: move set_mode() after mission.set_current_cmd()

if set_current_cmd() fails, we don't want to change to AUTO with some
unknown mission item
Plane: "Go around" operation. Assumes go around behavior follows land wp.
Plane: save a byte in auto_state

keep single bits together to save space on APM2
Plane: removed impossible boolean check
Plane: reset commanded_go_around at start of land

this allows multiple landing attempts
Plane: fixed verify_land() comment for go-around
Plane: removed incomplete DF log of go-around

the right commands should already be logged. This code would have
logged a lot of uninitialised bytes