stop chinook from returning to base with 'F' key

Rationale:
Annoying problem where chinook used to land when the F key was pressed, and then could unload with another F keypress. 
Now when you are trying delicately to land it, and fast, an accidental or early F key will send it home, usually over an enemy SAM site.
Changed the return to base key for helis to X. This was the original key in C&C / RA 95. Helis don't scatter or benefit from it anyway, so it makes sense to choose this key. Should not be F.