if available, use the path variable exposed from winsw.

See https://github.com/kohsuke/winsw/commit/2ffdbde6a92cf582baa4137e7b4de1116a3ca88b
Minor improvement to suggestion in Javadoc.
Removed horribly obsolete but apparently forgotten svn-password.jelly.
[FIXED JENKINS-22080] NPE if trying to install a plugin from the update center and either the update source or the plugin contains a '.' in its name