“You know you’re out of the business when you don’t recognize the names on ‘Hollywood Squares,’ ” Johnny Carson kidded Thursday as he celebrated his 78th birthday at his home in Malibu. He’s feeling fine, thank you, having fully recovered from bypass surgery almost five years ago. Of his age, he laughed, “They say we’re now playing with house money.” Carson said he spends his time on his boat and reading. Among those calling to wish him a happy birthday were his longtime friends producer Peter Lasally and former TV sidekick Ed McMahon. As for ever working again, he repeats his byword, “You have to know when to walk away.” Yeah, but we still miss him.