Always close fd's returned from internal.BPFProgLoad

[ti-mo]

There are a few places where internal.BPFProgLoad's return value is discarded, relying on the GC to close FDs using their finalizers. Relying on this is not ideal, since it's possible to run Go programs with GC disabled.

In newProgramWithOptions(), this is only done when re-running the prog load with verifier output enabled, but it's still theoretically possible for a retry to succeed and yield an fd. We should at least handle the fd if one happens to be returned.

In features/ this is also done in a few places, and it should definitely be handled correctly there. (@rgo3)