Fix var_InheritString leaks

Fix leaks caused by not cleaning up after var_InheritString.

The commits which mention Fixes: <sha> cannot be backported to 3.0.x, as these are all 4.0 only commits for various reasons.

The other commits can all be trivially backported (although it is questionable if d7f14cea should be), except for:

• 12c560e2 (would also need f00c62a8)
• 6760b65c (would require significant modifications)