AP_InertialNav: fix pos error degradation rate

degrate position_error to 10% over 2 seconds (assumes 5hz update rate)
must be *0.7943 instead of 0.7934
Copter: bug fix rally point landing target

was drifting toward home by some distance as (0,0,0) was hard-coded in
control_rtl.pde. Fixed, tested in SITL
Copter: ch7/8 switch for auto pause/continue

- added new mode for CH7/CH8 (#24, auto mission reset)
- changed mission.start() to mission.start_or_resume() in auto_init()