fixed paths in Doxyfile
changed semantic of `quit`, deprecated reply

this patch changes the semantics of `quit` slightly,
i.e., `quit` always guarantees that all currently running message
handlers are executed and the given exit reason is not applied
until `on_exit` ran an did not set a new behavior;
this patch also deprecates `reply`, because the recommended way
of replying to a message is to return the value from the
message handler