fdtable: add ioctl numbers for FIONBIO, FIONREAD

Rather than defining these constants in the POSIX layer,
define them in the Zephyr layer, and make the POSIX layer a
simple wrapper.

This will allow the POSIX layer and networking to have a
common dependency rather than a cyclic one.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>