Move value_true to value.h

I noticed that value_true is declared in language.h and defined in
language.c.  However, as part of the value API, I think it would be
better in one of those files.  And, because it is very short, I
changed it to be an inline function in value.h.  I've also removed a
comment from the implementation, on the basis that it seems obsolete
-- if the change it suggests was needed, it probably would have been
done by now; and if it is needed in the future, odds are it would be
done differently anyway.

Finally, this patch also changes value_true and value_logical_not to
return a bool, and updates some uses.

gdb/valarith.c

@@ -1653,9 +1653,9 @@ value_binop (struct value *arg1, struct value *arg2, enum exp_opcode op)
   return val;
 }
 
-/* Simulate the C operator ! -- return 1 if ARG1 contains zero.  */
+/* See value.h.  */
 
-int
+bool
 value_logical_not (struct value *arg1)
 {
   int len;