Get rid of explicit argument-count markings in tab-complete.c.

This replaces the "TailMatchesN" macros with just "TailMatches",
and likewise "HeadMatchesN" becomes "HeadMatches" and "MatchesN"
becomes "Matches".  The various COMPLETE_WITH_LISTn macros are
reduced to COMPLETE_WITH, and the single-item COMPLETE_WITH_CONST
also gets folded into that.  This eliminates a lot of minor
annoyance in writing tab-completion rules.  Usefully, the compiled
code also gets a bit smaller (10% or so, on my machine).

The implementation depends on variadic macros, so we couldn't have
done this before we required C99.

Andres Freund and Thomas Munro; some cosmetic cleanup by me.

Discussion: https://postgr.es/m/d8jo9djvm7h.fsf@dalvik.ping.uio.no

src/include/c.h

@@ -224,6 +224,39 @@
 #define CppAsString2(x)			CppAsString(x)
 #define CppConcat(x, y)			x##y
 
+/*
+ * VA_ARGS_NARGS
+ *		Returns the number of macro arguments it is passed.
+ *
+ * An empty argument still counts as an argument, so effectively, this is
+ * "one more than the number of commas in the argument list".
+ *
+ * This works for up to 63 arguments.  Internally, VA_ARGS_NARGS_() is passed
+ * 64+N arguments, and the C99 standard only requires macros to allow up to
+ * 127 arguments, so we can't portably go higher.  The implementation is
+ * pretty trivial: VA_ARGS_NARGS_() returns its 64th argument, and we set up
+ * the call so that that is the appropriate one of the list of constants.
+ * This idea is due to Laurent Deniau.
+ */
+#define VA_ARGS_NARGS(...) \
+	VA_ARGS_NARGS_(__VA_ARGS__, \
+				   63,62,61,60,                   \
+				   59,58,57,56,55,54,53,52,51,50, \
+				   49,48,47,46,45,44,43,42,41,40, \
+				   39,38,37,36,35,34,33,32,31,30, \
+				   29,28,27,26,25,24,23,22,21,20, \
+				   19,18,17,16,15,14,13,12,11,10, \
+				   9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
+#define VA_ARGS_NARGS_( \
+	_01,_02,_03,_04,_05,_06,_07,_08,_09,_10, \
+	_11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
+	_21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
+	_31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
+	_41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
+	_51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
+	_61,_62,_63,  N, ...) \
+	(N)
+
 /*
  * dummyret is used to set return values in macros that use ?: to make
  * assignments.  gcc wants these to be void, other compilers like char