count_nulls(VARIADIC "any")

Hi,

I'd like to suggest $SUBJECT for inclusion in Postgres 9.6.  I'm sure 
everyone would've found it useful at some point in their lives, and the 
fact that it can't be properly implemented in any language other than C 
I think speaks for the fact that we as a project should provide it.

A quick and dirty proof of concept (patch attached):

=# select count_nulls(null::int, null::text, 17, 'bar');
  count_nulls
-------------
            2
(1 row)

Its natural habitat would be CHECK constraints, e.g:

   CHECK (count_nulls(a,b,c) IN (0, 3))

Will finish this up for the next CF, unless someone wants to tell me how 
stupid this idea is before that.


.m