Document varadic args as int, since you can't have short varadic args (they are
promoted to ints).

• 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
Submitted by: Greg V at unrelenting
Differential Revision: https://reviews.freebsd.org/D21816