When specified, MUI calls this hook whenever a rexx message
arrives and MUI can't map it to a builtin or a programmer
specified command. The hook will be called with a pointer
to itself in A0, a pointer to the application object in A2
and a pointer to a struct RexxMsg in A1.
The return code from the hook is used as result code
when replying the message, the secondary result can
be set with MUIA_Application_RexxString.