Fixes mech sensors preventing shuttle undocking

Airlock controllers now use a signal_mech_sensor() proc to send commands
to mech sensors. Docking controllers don't appear to store the mech
sensor tag anywhere so the mech sensor code was left commented out.
Fixes escape shuttle console NanoUI error
Merge pull request #6408 from mwerezak/mech-sensor

Fixes mech sensors preventing shuttle undocking