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.
tests/bitops.0/test.sh:  change include dir to /include from /src/rtapi

Theory:  tests ought not to depend on anything in /src/
FIXME:  temp. don't kill realtime.in to debug a msgd hang

Instead get a gdb back trace and then hang forever.