ggate: fix test intermittency

[asomers]

Summary

• ggatel: fix piped output. Flush stdout before daemonizing, so the output can be read when it's in a command pipeline.

• ggated: bind the port _before_ daemonizing, so an invoking script can be sure that the port is bound as soon as the command returns.

• tests: allow ggatec or ggatel to automatically allocate a device number instead of choosing one in a racy way.

With all of these changes I can no run the tests 1000 times without
fail. And in parallel, too.

MFC after: 1 week
Sponsored by: ConnectWise
PR: 298942
Reported by: siva

Test Plan

existing test cases exercised

[ngie]

sbin/ggate/ggated/ggated.c

1123–1136

It might be a good idea to add a short comment noting the ordering.

tests/sys/geom/class/gate/ggate_test.sh

105–107

-s exit:0 is the default.