Implement llListSortStrided

The only difference between this function and llListSort is the index, so llListSort is adapted to accept it.

unit_tests/expr.suite/list-funcs-7.lsl

@@ -0,0 +1,9 @@
+  "T1" + llListSortStrided(["a", "á", "B", "C", "d", "e"], 1, 0, TRUE)
++ "T2" + llListSortStrided([1, "C", 3, "A", 2, "B"], 1, 0, TRUE)
++ "T3" + llListSortStrided([1, 3, 2, "C", "A", "B"], 1, 0, TRUE)
++ "T4" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, 0, TRUE)
++ "T5" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, 1, TRUE)
++ "T6" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, 2, TRUE)
++ "T7" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, -3, TRUE)
++ "T8" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, -2, TRUE)
++ "T9" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, -1, TRUE)

unit_tests/expr.suite/list-funcs-7.out

@@ -0,0 +1,52 @@
+[ "T1"
+, "B"
+, "C"
+, "a"
+, "d"
+, "e"
+, "á"
+, "T2"
+, 1
+, "A"
+, 2
+, "B"
+, 3
+, "C"
+, "T3"
+, 1
+, 2
+, 3
+, "A"
+, "B"
+, "C"
+, "T4"
+, 1
+, "C"
+, 2
+, "B"
+, 3
+, "A"
+, "T5"
+, 3
+, "A"
+, 2
+, "B"
+, 1
+, "C"
+, "T6"
+, "T7"
+, "T8"
+, 1
+, "C"
+, 2
+, "B"
+, 3
+, "A"
+, "T9"
+, 3
+, "A"
+, 2
+, "B"
+, 1
+, "C"
+]