add new set-external-build-result CLI command

With this command external job submission is possible per CLI.
Per pipe gzipped log file can be transported. No wrapper scripts are needed
any more to achive this per XML API from the command line:
cat log.gz | ssh jenkins set-external-build-result --job b --result 0 --log -

Sometimes build number is needed, as the calling program might need to put
an URL to the submitted build. This CLI command returns the new build number.