The Linux accept4() syscall allows two flags only: SOCK_NONBLOCK and
SOCK_CLOEXEC, and returns -EINVAL if any other bits have been set.
Change the qemu implementation accordingly, which means we can not use
the fcntl_flags_tbl[] translation table which allows too many other
values.
Beside the correction in behaviour, this actually fixes the accept4()
emulation for hppa, mips and alpha targets for which SOCK_NONBLOCK is
different than TARGET_SOCK_NONBLOCK (aka O_NONBLOCK).
The fix can be verified with the testcase of the debian lwt package,
which hangs forever in a read() syscall without this patch.