Add num_nulls() and num_nonnulls() to count NULL arguments.

An example use-case is "CHECK(num_nonnulls(a,b,c) = 1)" to assert that
exactly one of a,b,c isn't NULL.  The functions are variadic, so they
can also be pressed into service to count the number of null or nonnull
elements in an array.

Marko Tiikkaja, reviewed by Pavel Stehule

doc/src/sgml/func.sgml

@@ -182,7 +182,7 @@
   </sect1>
 
   <sect1 id="functions-comparison">
-   <title>Comparison Operators</title>
+   <title>Comparison Functions and Operators</title>
 
    <indexterm zone="functions-comparison">
     <primary>comparison</primary>
@@ -190,11 +190,11 @@
    </indexterm>
 
    <para>
-    The usual comparison operators are available, shown in <xref
-    linkend="functions-comparison-table">.
+    The usual comparison operators are available, as shown in <xref
+    linkend="functions-comparison-op-table">.
    </para>
 
-   <table id="functions-comparison-table">
+   <table id="functions-comparison-op-table">
     <title>Comparison Operators</title>
     <tgroup cols="2">
      <thead>
@@ -437,6 +437,49 @@
    </para>
 -->
 
+   <para>
+    Some comparison-related functions are also available, as shown in <xref
+    linkend="functions-comparison-func-table">.
+   </para>
+
+  <table id="functions-comparison-func-table">
+    <title>Comparison Functions</title>
+    <tgroup cols="4">
+     <thead>
+      <row>
+       <entry>Function</entry>
+       <entry>Description</entry>
+       <entry>Example</entry>
+       <entry>Example Result</entry>
+      </row>
+     </thead>
+     <tbody>
+      <row>
+       <entry>
+         <indexterm>
+          <primary>num_nonnulls</primary>
+         </indexterm>
+         <literal>num_nonnulls(VARIADIC "any")</literal>
+       </entry>
+       <entry>returns the number of non-NULL arguments</entry>
+       <entry><literal>num_nonnulls(1, NULL, 2)</literal></entry>
+       <entry><literal>2</literal></entry>
+      </row>
+      <row>
+       <entry>
+         <indexterm>
+          <primary>num_nulls</primary>
+         </indexterm>
+         <literal>num_nulls(VARIADIC "any")</literal>
+       </entry>
+       <entry>returns the number of NULL arguments</entry>
+       <entry><literal>num_nulls(1, NULL, 2)</literal></entry>
+       <entry><literal>1</literal></entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
   </sect1>
 
   <sect1 id="functions-math">
@@ -10389,7 +10432,7 @@ table2-mapping
   </note>
   <para>
    The standard comparison operators shown in  <xref
-   linkend="functions-comparison-table"> are available for
+   linkend="functions-comparison-op-table"> are available for
    <type>jsonb</type>, but not for <type>json</type>. They follow the
    ordering rules for B-tree operations outlined at <xref
    linkend="json-indexing">.