Guns N’ Roses, kicking off its first-ever headlining tour Friday night at Alpine Valley, negated the oft-repeated and seemingly true tale that today’s musicians have forgotten what rock ‘n’ roll is all about. Three years after its debut LP “Appetite for Destruction” clawed its way to the top of the album charts, the controversial Los Angeles band gave an aggressive, testosterone-laced performance before an almost sold-out crowd of 40,000 fans, showing that while its members’ tumultuous private lives and business idiosyncrasies are the stuff that keeps gossip columnists in business, their music contains all the elements that make rock ‘n’ roll vital.