No d’oh: “The Simpsons” on April 29 set the record for the most episodes of a prime-time scripted television series, surpassing the 635 tallied by the venerable 1955-1975 TV Western “Gunsmoke.” “This town ain’t big enough for the both of us,” declared “The Simpsons” Twitter page Friday as it previewed the episode’s “Gunsmoke”-themed opening credits.