Add a way to get the current function's OID in pl/pgsql.

Invent "GET DIAGNOSTICS oid_variable = PG_ROUTINE_OID".
This is useful for avoiding the maintenance nuisances that come
with embedding a function's name in its body, as one might do
for logging purposes for example.  Typically users would cast the
result to regproc or regprocedure to get something human-readable,
but we won't pre-judge whether that's appropriate.

Pavel Stehule, reviewed by Kirk Wolak and myself

Discussion: https://postgr.es/m/CAFj8pRA4zMd5pY-B89Gm64bDLRt-L+akOd34aD1j4PEstHHSVQ@mail.gmail.com

doc/src/sgml/plpgsql.sgml

@@ -1639,6 +1639,11 @@ GET DIAGNOSTICS integer_var = ROW_COUNT;
          <entry>line(s) of text describing the current call stack
           (see <xref linkend="plpgsql-call-stack"/>)</entry>
         </row>
+        <row>
+         <entry><literal>PG_ROUTINE_OID</literal></entry>
+         <entry><type>oid</type></entry>
+         <entry>OID of the current function</entry>
+        </row>
        </tbody>
       </tgroup>
      </table>