fix commander: run preflight checks on GCS connection
fix commander: run preflight checks on GCS connection

Regression from 6dec451babf1b4c6394fbf8678585d66932adefb, leading to
preflight failures not being reported at all. Only after a failed arming
attempt the messages would be sent. And for GPS check failures, in case
they are set to optional (default), arming would be possible, but switching
to position would be rejected w/o error.

We need to run the preflight checks periodically, but this at least restores
the previous behavior.

PX4/Firmware,krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware