To make it clearer to users when synchronization behaviour is not
supported, return ENOTSUP when O_RSYNC, O_DSYNC or O_SYNC are
respectively not defined. Linux also doesn't support O_RSYNC despite the
O_RSYNC flag being defined.
Return a WASI error code (rather than a host POSIX one). In addition,
there is no need to return an error in the case that the provided buffer
is too large.
The WASI docs allow for fewer rights to be applied to an fd than requested but
not more. This behavior is also asserted in the rust WASI tests, so it's necessary
for those to pass as well.
