Kernel+Userland: Implement setuid() and setgid() and add /bin/su

Also show setuid and setgid bits in "ls -l" output. :^)

Userland/su.cpp

@@ -0,0 +1,44 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <pwd.h>
+#include <grp.h>
+#include <alloca.h>
+
+extern "C" int main(int, char**);
+
+int main(int argc, char** argv)
+{
+    uid_t uid;
+    gid_t gid;
+    if (argc == 1) {
+        uid = 0;
+        gid = 0;
+    } else {
+        auto* pwd = getpwnam(argv[1]);
+        if (!pwd) {
+            fprintf(stderr, "No such user: %s\n", argv[1]);
+            return 1;
+        }
+        uid = pwd->pw_uid;
+        gid = pwd->pw_gid;
+    }
+
+    int rc = setgid(uid);
+    if (rc < 0) {
+        perror("setgid");
+        return 1;
+    }
+
+    rc = setuid(gid);
+    if (rc < 0) {
+        perror("setuid");
+        return 1;
+    }
+
+    rc = execl("/bin/sh", "sh", nullptr);
+    if (rc < 0) {
+        perror("execl");
+        return 1;
+    }
+    return 0;
+}