openat(2), mq_open(2): do not refer to mode_t for variadic arguments

[val_packett.cool]

Summary

• mode_t is uint16_t (sys/sys/_types.h)
• openat takes variadic args
• variadic args cannot be 16-bit, and indeed the code uses int
• the manpage currently kinda implies the argument is 16-bit by saying mode_t

Prompted by Rust things: https://github.com/tailhook/openat/issues/21

[imp]

LGTM