Also changed mode_allows_arming function to accept arming_from_gcs param
Also remove AUTOTUNE from arming list

Also initialise throttle_zero  to true on startup
Treat throttle less than zero as zero

this prevents a corrupted microSD card from causing a continuous
attempt to open a log file while in flight, which can cause large
scheduler delays

Pair-Programmed-With: Grant Morphett <grant@gmorph.com>

the fsync just wastes time reopening /fs/mtd

This enables user to set the external parameter to true even if only one compass is connected

dcm-check was triggering after 10 bad headings but these did not need to
be continuous meaning if the vehicle was flown long enough it would
almost certainly trigger a dcm-check failure and land