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.h

@@ -0,0 +1,16 @@
+#ifndef COSMOPOLITAN_THIRD_PARTY_MUSL_CRYPT_H_
+#define COSMOPOLITAN_THIRD_PARTY_MUSL_CRYPT_H_
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+struct crypt_data {
+  int initialized;
+  char __buf[256];
+};
+
+char *crypt(const char *, const char *);
+char *crypt_r(const char *, const char *, struct crypt_data *);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_THIRD_PARTY_MUSL_CRYPT_H_ */