common.xml: add CAPA indicating mission-slot-0 is home

[peterbarker]

ArduPilot stores home position in the first element of the mission.

Other autopilots do not.

Add a capability bit indicating that ArduPilot is doing this, which ArduPilot will set. Eventually this will allow ArduPilot to not store home in slot-0 - or at least not as far as the item transfer protocols are concerned (including FTP).

No new autopilot should set the bit.

The concept is:

• ArduPilot starts to send this bit
• GCSs start to understand this bit, and will do the "slot-0-is-home" if the bit is set, as well as the existing special case for ArduPilot.
• After ArduPilot has been sending this bit for a prolonged period of time GCSs can remove the special-case for ArduPilot, relying on the bit.
• After the GCSs can have been expected to support the bit for some prolonged period of time ArduPilot can stop setting the bit.
• After even more time ArduPilot can stop setting the bit and start transferring the missions without home in them
• eventually everyone is transferring missions without home in them and we can deprecate the bit entirely as unused

Drawback: the mavlink node initiating transfer MUST know the MAV_PROTOCOL_CAPABILITY mask of the autopilot before it can start a mission transfer

[peterbarker]

UTC1103 - https://github.com/ArduPilot/mavlink/pull/360
Could do a special NAV_HOME mission item type too
Parsing tlogs - don’t have capability bits so you don’t know what home is
GCSs which can’t read capabilities can’t work (e.g. read-only GCSs)
Still don’t know whether to add home at slot 0
Probably need the capability flags
Introduce NAV_HOME at the same time
Can NAV_HOME be anywhere than slot-0?
Nav_home an be displayed specially on the map
Will come back to it next week