Use newDFunName for both manual and derived instances (#17339)
Issue #17339 (closed) was caused by using a slightly different version of
newDFunName
for derived instances that, confusingly enough, did not
take all arguments to the class into account when generating the
DFun
name. I cannot think of any good reason for doing this, so
this patch uses newDFunName
uniformly for both derived instances
and manually written instances alike.
Fixes #17339 (closed).