mka: Fix multiple key server election bugs

mka: Fix multiple key server election bugs

1. The comparison between SCI's of two servers with identical priority
   is broken, and would always return TRUE. Just use os_memcmp(), which
   provides the ordering we need.

2. If no peer can be key server but this instance can, then become the
   key server.

3. The ordering of blocks between peer as key server and ourself as key
   server overwrites settings. Simple reordering fixes this.

4. Default to being the key server, so that we advertise our ability in
   the MKPDUs we send. That's the only way peers can know we can be key
   server. Cleared automatically as soon as we find a better peer.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>