enhancement: return to mission after GPS glitch recovery

i would like to suggest an enhancement for the vehicle to either resume or restart AUTO mode mission after recovering from a GPS_GLITCH event. Right now the only option upon GPS_GLITCH is failsafe to land or althold, but with more autonomous flying going on in remote fields where visibility is not always best it would be very beneficial for the copter to have more options when GPS function (HDOP) is restored, and one is to continue mission at nearest waypoint or to restart from first waypoint would help those who are flying farther away rather than the copter just siting there althold or land somewhere dangerous.
there is already function on AC3.2.1 to resume AUTO mode mission from beginning when switched out into LOITER or STABILIZE and back to AUTO, so a lot of the capability already exists in some sort of fashion.

Start by reviewing the existing GPS_GLITCH failsafe and AUTO mission recovery behavior, then compare it with the AC3.2.1 behavior described in the issue. Done should include a defined and implemented choice between resuming at the nearest waypoint or restarting from the first waypoint after GPS recovery, with appropriate validation.

Written by the indexing model from the issue text.