18.104.22.168: fix a long-standing UNINTERN bug
Patch by Stas Boukarev, lp#693796.
UNINTERN is specified to take a symbol, not a symbol
designator: if P1 has the symbol P1:S, which is not EQ
to P2:S, then (UNINTERN 'P2:S :P1) should not remove
P1:S from P1.