Fix defmethod arglists leaking into make-method-lambda.

Fix defmethod arglists leaking into make-method-lambda.

defmethod communicates to make-method-lambda using special variables,
but make-method-lambda then performs code-walking which expands
macros, and if a macro calls another make-method-lambda directly, this
make-method-lambda will receive incorrect information which may cause
problems with wrong lambda lists.
Reported and diagnosed by Attila Lendvai.