net: sockets: support fionbio and fionread

The `ioctl()` interface is already supported by the network
subsystem but there was no `zsock_` interface available for it.

Add the `zsock_ioctl()` syscall.

Implement two somewhat commont ioctl requests for socket
file descriptors; namely

- `FIONBIO` set non-blocking I/O mode
- `FIONREAD` get the number of available bytes

In the process, added `net_pkt_ip_proto_hdr_len()`

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