net: lwm2m: Fix native_posix fcntl() calls

Native POSIX target still needs to use the <fcntl.h> header
instead of <zephyr/posix/fcntl.h>

Also removed the include from various files that did not use it.

Also changed fcntl() calls to zsock_fcntl() because we directly
use zsock_* calls elsewhere.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>