JAVA_HOME auto discovery

When JAVA_HOME is not set (but java command is in the PATH) then bin/mvn script prints warning message "Warning: JAVA_HOME environment variable is not set."
I don't want to explicitly set JAVA_HOME variable because I manage my JDK instances by update-java-alternatives in Ubuntu. It switches system wide JDK, but does not set the JAVA_HOME variable. If I would set JAVA_HOME manually then any changes made by update-java-alternatives would not be reflected in JAVA_HOME which is obviously incorrect.

However mvn script can be improved, so that it can guess JAVA_HOME. Actually Groovy startup script does it very well, so I moved auto-discovery code from groovy script to mvn and attach patch here