Cannot create an aggregate function with variadic parameters and enabled for parallel execution

When trying to improve my extension Argm to work with parallel queries I
found that
CREATE AGGREGATE wants serial/deserial/combine functions
to have variadic any parameter if the aggregate function itself is
variadic any.
If I add the requested parameter, CREATE AGGREGATE still cannot find the
function.

Having an analog of [m]finalfunc_extra for serial/deserial/combine functions
may be a good idea, but it's not yet there. So the attached patch makes
postgres
skip the check for the parallel support functions.

I would appreciate if someone could have a look at the patch.
The problem is probably actual for 9.6+, so back-patch would be nice if
there's no objections.

Should one be curious how to reproduce the problem, please clone
https://github.com/bashtanov/argm/tree/parallel
and try make && make install && make installcheck. This fails without
the patch, but works with it.
The error message is "function argmax_combine(internal, internal) must
accept VARIADIC ANY to be used in this aggregate".
(Just in case: this "parallel" branch is a WIP, please use "master"
branch if you need something stable).

Best regards,
  Alexey