Replace my misguided "java" command with "javav".

javav has two purposes:

`source javav <version>` modifies your JAVA_HOME and PATH environment
variables to point to the specified version.

`javav <version> <command>` runs the specified command in the context of
the specified Java version by changing the JAVA_HOME and PATH variables,
but only for the specified command. It doesn't change your running shell
environment.
Better version detection in javav, including failure.