net: socket: mgmt: Create proper socket options for net_mgmt sockets

We cannot use the network management event number directly as
a socket option value because the management value is uint64_t
and that cannot be mapped directly to 32 bit integer.
So have an intermediate socket option that is mapped to actual
network management request number in getsockopt() and setsockopt().

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>