Kit: Adapter: use more reliable utun detection technique

Rather than hoping that the AF_SYSTEM fd is of type utun, and then
calling "2" on it to get the name -- which could be defined as something
else for a different AF_SYSTEM socket type -- instead simply query the
AF_SYSTEM control socket ID with getpeername. This has one catch, which
is that the ID is dynamically allocated, so we resolve it using the
qualified name. Normally we'd make a new AF_SYSTEM socket for this, but
since that's not allowed in the sandbox, we reuse the AF_SYSTEM socket
that we're checking. At this point in the flow, we know that it's a
proper AF_SYSTEM one, based on the first sockaddr member; we just don't
know that it's a utun variety.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Sources/WireGuardKitC/WireGuardKitC.h

@@ -3,3 +3,18 @@
 
 #include "key.h"
 #include "x25519.h"
+
+/* From <sys/kern_control.h> */
+#define CTLIOCGINFO 0xc0644e03UL
+struct ctl_info {
+    u_int32_t   ctl_id;
+    char        ctl_name[96];
+};
+struct sockaddr_ctl {
+    u_char      sc_len;
+    u_char      sc_family;
+    u_int16_t   ss_sysaddr;
+    u_int32_t   sc_id;
+    u_int32_t   sc_unit;
+    u_int32_t   sc_reserved[5];
+};