Add test for ioctl(SIOCGIFCONF) and polyfill on BSDs

- Use nullness checks when calling weakly linked functions.

- Avoid typedef for reasons described in Linux Kernel style guide.

- Avoid enum in in Windows headers. Earlier in Cosmo's history all one
  hundred files in libc/nt/enum/ used to be enums and it resulted in
  gigabytes of DWARF data almost as large as everything else in the
  codebase combined.

- Bitfields aren't our friends. They have frequent ABI breakages,
  inconsistent arithmetic across compilers, and different endianness
  between cpus. Compiler authors also haven't invested much roi into
  making bit fields go fast so they produce poor assembly.

- Use memccpy() instead of strncpy() or snprintf() for length-bounded
  copying of C strings. strncpy() is a misunderstood function and
  snprintf() is awesome but memccpy() deserves more love.

libc/calls/struct/sockaddr6.h

@@ -0,0 +1,22 @@
+#ifndef COSMOPOLITAN_LIBC_CALLS_STRUCT_SOCKADDR6_H_
+#define COSMOPOLITAN_LIBC_CALLS_STRUCT_SOCKADDR6_H_
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+
+struct in6_addr {
+  union {
+    uint8_t s6_addr[16];
+    uint16_t s6_addr16[8];
+    uint32_t s6_addr32[4];
+  };
+};
+
+struct sockaddr_in6 { /* Linux+NT ABI */
+  uint16_t sin6_family;
+  uint16_t sin6_port;
+  uint32_t sin6_flowinfo;
+  struct in6_addr sin6_addr;
+  uint32_t sin6_scope_id; /* rfc2553 */
+};
+
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_CALLS_STRUCT_SOCKADDR6_H_ */