Disallow operator!() on unique_fd

This catches a common mistake where client code checks for errors using
the common idiom that works for std::iostream and other file-like
classes:

  unique_fd fd = open(...);
  if (!fd) {
  }

Test: atest libbase_test
Test: m droid
Change-Id: I9629a7795537ecb3b57be9c741c06f80967e4cc2