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.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"
+]