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/sock.h

@@ -102,11 +102,11 @@ struct ifconf {
 #define IFHWADDRLEN 6
 #define IF_NAMESIZE 16
 #define IFNAMSIZ    IF_NAMESIZE
+
 struct ifreq {
   union {
     char ifrn_name[IFNAMSIZ]; /* Interface name, e.g. "en0".  */
   } ifr_ifrn;
-
   union {
     struct sockaddr ifru_addr;      /* SIOCGIFADDR */
     struct sockaddr ifru_dstaddr;   /* SIOCGIFDSTADDR */
@@ -128,8 +128,6 @@ struct ifreq {
 #define _IOT_ifreq_short _IOT(_IOTS(char), IFNAMSIZ, _IOTS(short), 1, 0, 0)
 #define _IOT_ifreq_int   _IOT(_IOTS(char), IFNAMSIZ, _IOTS(int), 1, 0, 0)
 
-#define IFF_UP (1 << 0)
-
 const char *inet_ntop(int, const void *, char *, uint32_t);
 int inet_aton(const char *, struct in_addr *);
 int inet_pton(int, const char *, void *);