fmgr.h: PG_NARGS() for number of arguments passed

The small context diff below corrects what seems to be an oversight in
fmgr.h - it's discouraged to access fcinfo directly but there is no
macro to get the number of arguments passed to the function. Checking
the number of arguments is often useful when you have a function which
can be called like:

 func('arg');
 func(null);
 func();

all mapping to the same C function.

the marco has a function-like appearance to match the other PG_*
macros.

Regards, Lee Kindness.

Index: fmgr.h
===================================================================
RCS file: /projects/cvsroot/pgsql-server/src/include/fmgr.h,v
retrieving revision 1.24
diff -c -r1.24 fmgr.h
*** fmgr.h    2002/09/04 20:31:36    1.24
--- fmgr.h    2002/09/10 13:25:06
***************
*** 116,121 ****
--- 116,126 ----
  #define PG_FUNCTION_ARGS    FunctionCallInfo fcinfo

  /*
+  * Get number of arguments passed to function.
+  */
+ #define PG_NARGS() (fcinfo->nargs)
+
+ /*
   * If function is not marked "proisstrict" in pg_proc, it must check for
   * null arguments using this macro.  Do not try to GETARG a null argument!
   */