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/sock/gethostips.c

@@ -59,8 +59,8 @@ static uint32_t *GetUnixIps(void) {
 
 static textwindows uint32_t *GetWindowsIps(void) {
   uint32_t i, z, n, rc, *a;
-  NtIpAdapterUnicastAddress *u;
-  NtIpAdapterAddresses *p, *ifaces;
+  struct NtIpAdapterUnicastAddress *u;
+  struct NtIpAdapterAddresses *p, *ifaces;
   i = 0;
   z = 15000;
   do {