Add ioctl(SIOCGIFxxx) support (#197)

- SIOCGIFCONFIG: reads and enumerate all the network interfaces
- SIOCGIFADDR: reads network address for a given interface
- SIOCGIFFLAGS: reads network flags for a given interface
- SIOCGIFNETMASK: reads network netmask for a given interface
- SIOCGIFBRDADDR: reads network broadcast address for a given interface
- SIOCGIFDSTADDR: reads peer destination address for a given
  interface (not supported for Windows)

This change defines Linux ABI structs for the above interfaces and adds
polyfills to ensure they behave consistently on XNU and Windows.

libc/sock/internal.h

@@ -53,6 +53,45 @@ struct sockaddr_un_bsd {
   char sun_path[108];
 };
 
+/* -----------------------------------------------------------------------------------
+ * ioctl SIOCGIFCONF & others:
+ *
+ * BSD has a different structure. All the ioctl will adjust to use
+ * the ifreq with the ifreq_bsd
+ */
+struct ifreq_bsd {
+  union {
+    char ifrn_name[IFNAMSIZ];
+  } ifr_ifrn;
+
+  union {
+    /* Right now we only list the structures used by the few ioctl that are
+     * supported by Cosmopolitan. Add your own definition here if you need
+     * a particular ioctl that requires polyfill for BSD
+     */
+    struct  sockaddr_bsd ifru_addr;
+    struct  sockaddr_bsd ifru_netmask;
+    struct  sockaddr_bsd ifru_dstaddr;
+    struct  sockaddr_bsd ifru_broadaddr;
+    short   ifru_flags;
+    char    ifru_pad[16];   /* used as padding */
+  } ifr_ifru;
+};
+
+#pragma pack(4)
+struct ifconf_bsd {
+  uint32_t ifc_len;            /* size of buffer   */
+  union {
+    char *ifcu_buf;
+    struct ifreq_bsd *ifcu_req;
+  } ifc_ifcu;
+};
+#pragma pack()
+
+
+/* ------------------------------------------------------------------------------------*/
+
+
 struct SockFd {
   int family;
   int type;