libthr: avoid varargs in fcntl and openat interposers

Align these signatures with the ones in syscalls.master (and thus
libsys.h). There's no reason to do va_args twice and in some ABIs
(e.g,, CheriABI) you can't access fixed arguments as varargs if you
weren't called with varargs signature.

Reviewed by: imp, kib, jhibbits
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D45126