DR fails to deliver directly-sent alarm signal if app has no itimer

I stumbled on this bug while writing a test for #4998 (closed). If an app invokes kill(x,SIGPROF), but that app has no itimer, DR fails to deliver the signal. DR's itimer-multiplexing logic assumes the signal is part of an itimer.

As an extension, if the app has an itimer and also directly sends an alarm signal, DR will treat the direct one as an itimer as it can't tell the difference: but that shortcoming we can probably live with. The case of no itimer is solvable and should be solved.