FRIDAY HARBOR, San Juan County — The people of the San Juan Islands tend to be independent sorts, espousing a do-it-yourself, leave-me-be ethos as natural and ever-present as the tide. But for many of the 17,000 people of this island county, the normal rhythms of small-town life have hit a dissonant chord lately. A couple of months ago, the U.S. Border Patrol began occasional "spot checks" of every vehicle and passenger arriving in Anacortes off state ferries, the lifeline between these islands and the mainland. For some here, it seems like a good idea or, at worst, a minor inconvenience....