unix: add a simple test to check the flags
unix: add a simple test to check the flags

Check basic bit conflicts by comparing the sum and the or of all known
flags. Add a regression test for #896.

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>

carllerche/mio,murarth/mio,carllerche/mio