Ignore (from)Net time on positions with an unknown or fixed location source

[thebentern]

Idea here for location sourcing is:

1. Clients (phone apps) with quality GPS position packets set location source of EXTERNAL
2. GPS.cpp sets its position with location source of INTERNAL
3. A node with fixed_position set gets alloc'd as location source of MANUAL

When considering a position packet as a potential time source of RTCQualityFromNet, the firmware will then ignore any position with a location source of UNKNOWN or MANUAL.