unique_fd's implicit conversion to int allows it to be passed to
close(2), which is dangerous because unique_fd will think that it still
has ownership of the now-closed fd. Disallow this by providing an
overload for close that's tagged with an attribute that gives a
compile-time error.
Test: m
Change-Id: I514591335b337f2f57c1df371cf3979304aea17c
6496c4bf6f