Make more libc improvements

- Make memmem() faster
- Make readdir() thread safe
- Remove 64kb limit from mkdeps.com
- Add old crypt() function from Musl
- Improve new fix-third-party.py tool
- Improve libc/isystem/ headers and fix bugs

third_party/musl/crypt_des.internal.h

@@ -0,0 +1,16 @@
+// clang-format off
+#ifndef CRYPT_DES_H
+#define CRYPT_DES_H
+
+#include "libc/limits.h"
+#include "libc/literal.h"
+
+struct expanded_key {
+	uint32_t l[16], r[16];
+};
+
+hidden void __des_setkey(const unsigned char *, struct expanded_key *);
+hidden void __do_des(uint32_t, uint32_t, uint32_t *, uint32_t *,
+                     uint32_t, uint32_t, const struct expanded_key *);
+
+#endif