Clean up more code

The *NSYNC linked list API is good enough that it deserves to be part of
the C libray, so this change writes an improved version of it which uses
that offsetof() trick from the Linux Kernel. We vendor all of the *NSYNC
tests in third_party which helped confirm the needed refactoring is safe

This change also deletes more old code that didn't pan out. My goal here
is to work towards a vision where the Cosmopolitan core libraries become
less experimental and more focused on curation. This better reflects the
current level of quality we've managed to achieve.

libc/intrin/dll.h

@@ -0,0 +1,57 @@
+#ifndef COSMOPOLITAN_LIBC_INTRIN_DLL_H_
+#define COSMOPOLITAN_LIBC_INTRIN_DLL_H_
+#ifdef COSMO
+#define dll_make_first   __dll_make_first
+#define dll_make_last    __dll_make_last
+#define dll_remove       __dll_remove
+#define dll_splice_after __dll_splice_after
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+#define DLL_CONTAINER(t, f, p) ((t *)(((char *)(p)) - offsetof(t, f)))
+
+struct Dll {
+  struct Dll *next;
+  struct Dll *prev;
+};
+
+static inline void dll_init(struct Dll *e) {
+  e->next = e;
+  e->prev = e;
+}
+
+static inline int dll_is_empty(struct Dll *list) {
+  return !list;
+}
+
+static inline struct Dll *dll_last(struct Dll *list) {
+  return list;
+}
+
+static inline struct Dll *dll_first(struct Dll *list) {
+  struct Dll *first = 0;
+  if (list) first = list->next;
+  return first;
+}
+
+static inline struct Dll *dll_next(struct Dll *list, struct Dll *e) {
+  struct Dll *next = 0;
+  if (e != list) next = e->next;
+  return next;
+}
+
+static inline struct Dll *dll_prev(struct Dll *list, struct Dll *e) {
+  struct Dll *prev = 0;
+  if (e != list->next) prev = e->prev;
+  return prev;
+}
+
+void dll_remove(struct Dll **, struct Dll *) paramsnonnull() libcesque;
+void dll_make_last(struct Dll **, struct Dll *) paramsnonnull((1)) libcesque;
+void dll_make_first(struct Dll **, struct Dll *) paramsnonnull((1)) libcesque;
+void dll_splice_after(struct Dll *, struct Dll *) paramsnonnull((1)) libcesque;
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMO */
+#endif /* COSMOPOLITAN_LIBC_INTRIN_DLL_H_ */