Messages: merge macros with and without message code

Previously we had two types of macros for logging: with and without
message code. They were intended to be merged afterwards.

The idea is to use a special code – `XKB_LOG_MESSAGE_NO_ID = 0` – that
should *not* be displayed. But we would like to avoid checking this
special code at run time. This is achieved using macro tricks; they
are detailed in the code (see: `PREPEND_MESSAGE_ID`).

Now it is also easier to spot the remaining undocumented log entries:
just search `XKB_LOG_MESSAGE_NO_ID`.