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>

include/zephyr/sys/fdtable.h

@@ -166,6 +166,10 @@ enum {
 	ZFD_IOCTL_POLL_UPDATE,
 	ZFD_IOCTL_POLL_OFFLOAD,
 	ZFD_IOCTL_SET_LOCK,
+
+	/* Codes above 0x5400 and below 0x5500 are reserved for termios, FIO, etc */
+	ZFD_IOCTL_FIONREAD = 0x541B,
+	ZFD_IOCTL_FIONBIO = 0x5421,
 };
 
 #ifdef __cplusplus