Arthur Kopit, the young Harvard graduate whose play Oh Dad, Poor Dad, Mamma's Hung You in the Closet and I'm Feeling So Sad is one of off-Broadway's most durable current hits, once wrote a play called Aubade. But he is not the sort of fellow who would make that mistake twice. His middle period, when he was about 21 a few months back, was notable for a three-acter called On the Runway of Life You Never Know What's Coming Off Next, a play that unfortunately contained its entire essence in the title and was never professionally produced. But now Kopit has prepared a new script for production next month off-Broadway, and Dad-wise he is only half a dozen picas off the peak. The new play, scheduled to go into Greenwich Village's Theatre de Lys, is called Asylum or What the Gentlemen Are Up To Not to Mention the Ladies.