SanityCheck: prevent unsafe forms of arming

Arming while in autonomous modes (e.g. RTH, PH, AH) can
allow unexpected behavior and is certainly an untested
path. This patch creates an error condition when in those
modes and disarmed or arming to prevent arming. In this
case one should switch to a traditional manual mode, arm,
and then engage the AP functionality.
Merge pull request #1004 from peabody124/safe_arming

SanityCheck: prevent unsafe forms of arming