WIP: libei: add an optional 64-bit UID to the device

Where we're running in a sandbox, having to ack/nak every device on demand is tiring. An easier approach would be to teach the clients to create a static UID for a device, have that acked once by the user and be able to re-use the same device in the future.

Obviously this requires some trust in the client to not mess with things, but where the sandbox remembers the capability set for a device in addition to the UID there isn't really a difference to just prompting anyway.

This is a 64 bit ID as it allows the prospect of making the device + client + capability set into a fairly simple 128 UUID, e.g. first byte is a bitmask of capabilities, 120 bits of client ID based on (maybe) the app-id and then the 128 bits of device UID. UUID4 support is wide-spread, so fitting into that makes things easy.

cc @jadahl, lemme know what you think please

WIP: because this is just the libei setter/getter, nothing else yet