Copter: arming allowed within fence margin

With a polyfence (and possibly tin can?) if you are within fence margin of the fence you can arm but the copter will immediately tilt hard to get out of the margin region. This led to tipovers on 3 quads in testing till we worked out the cause.
example log:
http://uav.tridgell.net/tmp/copter-fence-arm-margin.bin

Start with the provided copter fence-margin log and trace the arming and fence-margin behavior in the ArduCopter sources. Confirm the conditions that allow arming inside the margin, then define done as preventing the immediate hard tilt when armed in that position and covering the behavior with a regression test.

Written by the indexing model from the issue text.