directly mapped to the posix method sem_trywait

It currently cannot be configured via mavlink mission protocol.

Since the altitude is not used, we can ignore this as well.

Since vertex_count is in a union with do_jump_current_count, we can't
always reset the current count, otherwise the vertex_count ends up being
0.

This also removes the 'navigator fence <lon> <lat>' command to simplify
code (I don't think there's still use for that anymore). However the
file loading is still supported.

If goefence.txt does not exist, navigator will not clear the geofence
anymore on startup.

... and init to 0 is not needed, since already done in constructor

- retrieve & store the geofence & rally point data from/to dataman
- interleaved transmissions (of different types) are not possible. trying
  to do so will NACK the new transmission
- only one storage backend for polygons & rally points (not alternating
  between 2 as the mission does)

- orb_advertise already publishes a struct, no need for orb_publish
- mavlink_log_critical goes to the console too

- this was never read
- it was implemented wrong, leading to memory access violations in
  publishFence (an integer was passed instead of the fence_s struct)