Added new array APIs that also work with non-arrays.

Similar to how C pointers to variables can always be considered as
pointers to the first element of an array of size 1 (see the
following code for an example of how they are equivalent),
treating non-NXT_CONF_VALUE_ARRAY as if they were
NXT_CONF_VALUE_ARRAYs of size 1 allows for simpler and more
generic code.

	void foo(ptrdiff_t sz, int arr[sz])
	{
		for (ptrdiff_t i = 0; i < sz; i++)
			arr[i] = 0;
	}

	void bar(void)
	{
		int  x;
		int  y[1];

		foo(1, &x);
		foo(1, y);
	}

nxt_conf_array_elements_count_or_1():
	Similar to nxt_conf_array_elements_count().
	Return a size of 1 when input is non-array, instead of
	causing undefined behavior.  That value (1) makes sense
	because it will be used as the limiter of a loop that
	loops over the array and calls
	nxt_conf_get_array_element_or_itself(), which will return
	a correct element for such loops.

nxt_conf_get_array_element_or_itself():
	Similar to nxt_conf_get_array_element().
	Return the input pointer unmodified (i.e., a pointer to
	the unique element of a hypothetical array), instead of
	returning NULL, which wasn't very useful.

nxt_conf_array_qsort():
	Since it's a no-op for non-arrays, this API can be reused.

src/nxt_conf.c

@@ -392,6 +392,13 @@ nxt_conf_array_elements_count(nxt_conf_value_t *value)
 }
 
 
+nxt_uint_t
+nxt_conf_array_elements_count_or_1(nxt_conf_value_t *value)
+{
+    return (value->type == NXT_CONF_VALUE_ARRAY) ? value->u.array->count : 1;
+}
+
+
 nxt_uint_t
 nxt_conf_type(nxt_conf_value_t *value)
 {
@@ -750,6 +757,25 @@ nxt_conf_get_array_element(nxt_conf_value_t *value, uint32_t index)
 }
 
 
+nxt_conf_value_t *
+nxt_conf_get_array_element_or_itself(nxt_conf_value_t *value, uint32_t index)
+{
+    nxt_conf_array_t  *array;
+
+    if (value->type != NXT_CONF_VALUE_ARRAY) {
+        return (index == 0) ? value : NULL;
+    }
+
+    array = value->u.array;
+
+    if (index >= array->count) {
+        return NULL;
+    }
+
+    return &array->elements[index];
+}
+
+
 void
 nxt_conf_array_qsort(nxt_conf_value_t *value,
     int (*compare)(const void *, const void *))