TetherController: Use correct exit() after fork()+failed exec().

The bluetooth hack I98a1c8aedfa042b1824de7d8c69f2600c9bfcf29 uses the
wrong exit() after the exec() fail.
This will have undesirable effects like cpp destructors and atexit() funcs
being invoked which could have undesirable effects for the parent.
Also fixed other prior instance of fork()+exec()+"return"

Change-Id: I8c09472a371640a710c55fc18e020b9c152bf9cc