This commit moves the check that determines whether the mode
argument of `open*()` exists into a separate function.
With that, the check is fixed because previously it failed to
account for the fact that `O_TMPFILE` is not a power of two.
Furthermore, add `assert()`s in the fortified variants that
ensure that no mode is required by the specified flags.
_FORTIFY_SOURCE defines always_inline versions of open etc. functions,
which conflict with our exported override versions. Disable it for the
file that defines the override functions.
Keep separate map for fd to files so we can implement dup.
Filter out duplicates in enumfmt.
tryfmt should return the closest match.
Use pthread_once to init the global state.
Make things mostly work in GStreamer.
