net: mgmt: Convert the mgmt API to use 64-bit masks

Instead of using 32 bit enum values for event numbers, convert
the code to use 64 bit long bit fields. This means that the
user API is changed to use 64 bit event values instead of 32
bit event values.

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