Implement enum_style=java option.

This javanano_out command line option creates a container interface
at the normal place where the enum constants would reside, per enum
definition. The java_multiple_files flag would now affect the file-
scope enums with the shells. If the flag is true then file-scope
container interfaces are created in their own files.

Change-Id: Id52258fcff8d3dee9db8f3d8022147a811bf3565