util-device.h exposes some details that are particularly problematic
for C++, even when wrapped in 'extern "C"'. To address this, break the
header into public and private parts. The public part exposes
LiveDevice as an opaque data structure, while the private header has
the actual definition.
The idea is that only Suricata C source files should include the
private header, it should not be re-included in any other header
file. And this is the header library users should use, however we
don't enforce it with tecnical means, a library user could still
include the private header, but the clue there is in the name.
This can happen when the flow is UDP, but an ICMP unreachable is
returned, which gets assigned to the same flow.
Reference: https://github.com/ntop/nDPI/issues/2762
Split DetectHelperKeywordRegister into 2 functions, one for acquiring
a new keyword ID, and another to perform the registration.
This makes it easier to do the traditional C keyword initialization
with a dynamic ID.