os: wrap accept4(SOCK_CLOEXEC)

Some system C libraries do not have SOCK_CLOEXEC, and completely miss
accept4(), too. Provide a fallback for this case.

This changes the behaviour: no error messages are printed now for
failing to set CLOEXEC but the file descriptor is closed.

The unit test for this wrapper is NOT included.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>