don't flip the ship for main-thruster deceleration during coast phase

As requested on the forum:
http://pioneerspacesim.net/forum/viewtopic.php?f=3&t=198

(and possibly in other places)
Initialize m_hyperspace on load (hopefully fixes #2847).
Disable the save button when in hyperspace
Merge branch 'disable_hyperspace_save'
Changelog
Merge remote-tracking branch 'jpab/defer-retro-turn'