lib: use `spawn` options for timeout in `execFile`

Timeout support was added to spawn in v15.13.0 (PR), but execFile has continued to use its own mechanism to implement the timeout functionality, which is now basically replicating the logic added to spawn.

This PR removes the repeated logic from execFile and delegates timeout handling to spawn instead. It does so by forwarding the options timeout and killSignal.

The only visible change I can think of is a different order of parameter validation in execFile, which means a possibly different error message in the case that both timeout and at least another option are passed invalid values. Looking at the other functions in child_process and their tests, and after reading the documentation, it seems to me though that no particular order of parameter validation is being enforced, so I guess this change would have no significant impact.

No functionality was added or removed. All tests are still passing.