The internal client asserts that the length of the client ID is not more
than MAX_CLIENT_ID_LEN. Avoid that assert by truncating the string.

Also add new nm_dhcp_client_set_client_id_*() setters, that either
set the ID based on a string (in our common dhclient specific
format), or based on the binary data (as obtained from systemd client).

Also, add checks and assertions that the client ID which is
set via nm_dhcp_client_set_client_id() is always of length
of at least 2 (as required by rfc2132, section-9.14).