Adding ToolInstaller.preferredLocation and using from existing installers.
Validating ToolInstallation.home if it is configured, but OK if blank.
Hacking JavaScript to display ToolInstallation.home field in UI
only if "Install automatically" unchecked (or field is not blank).
Simplifying parameters of DownloadFromUrlInstaller.isUpToDate.
chmod a+x after DownloadFromUrlInstaller unpacks (needed for Ant/Maven installations on Unix).

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@18572 71c3de6d-444a-0410-be80-ed276b4c234a