libthr: avoid varargs in fcntl and openat interposers

[brooks]

Summary

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.

Obtained from: CheriBSD

[jhibbits]

LGTM, though I didn't compile it.