EN CONCERTWhether you love Jack Johnson, or find his amiable island charm a little too mild for your post modern comfort, props must be given for his confidently re creating a niche for acoustic singer songwriter fare that had its heyday in the '70s. Not since then has a young artist seemed so comfortable projecting a sunny, angst free persona, in service of a folk derived, heart centered musical agenda. His legions of multi generational fans suggest that the