8166568: Add jmod extract subcommand 8169492: jdk.internal.jmod.JmodFile.JMOD_MAGIC_NUMBER is a mutable array Reviewed-by: alanb, anazarov, dfuchs, mchung

^build/
^dist/
^.idea/
nbproject/private/
^webrev
^.src-rev$
^.jib/
.DS_Store
.metadata/
.recommenders/