id: Use unveil()

And so "id" becomes our first user of unveil(), giving himself access
to read /etc/passwd and /etc/group :^)

Userland/id.cpp

@@ -40,6 +40,21 @@ static bool flag_print_gid_all = false;
 
 int main(int argc, char** argv)
 {
+    if (unveil("/etc/passwd", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    if (unveil("/etc/group", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    if (unveil(nullptr, nullptr) < 0) {
+        perror("unveil");
+        return 1;
+    }
+
     if (pledge("stdio rpath", nullptr) < 0) {
         perror("pledge");
         return 1;