msgd: use synchronous signal delivery with signalfd()

this is the way it will be done with the zeroMQ zpoll() reactor, and
it avoids a problem with syslog() use from a signal hamdler.
rtapi/msgd: explicitly shut down rtapi_app if exiting

there is no point in rtapi_app continuing if rtapi_msgd exits for some reason.
This closes the situation where rtapi_msgd exits and rtapi_app hangs
around, giving troubles on next startup.
FIXME:  temp. don't kill realtime.in to debug a msgd hang

Instead get a gdb back trace and then hang forever.