Implement basic chmod() syscall and /bin/chmod helper.

Only raw octal modes are supported right now.
This patch also changes mode_t from 32-bit to 16-bit to match the on-disk
type used by Ext2FS.

I also ran into EPERM being errno=0 which was confusing, so I inserted an
ESUCCESS in its place.

Userland/chmod.cpp

@@ -0,0 +1,27 @@
+#include <unistd.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <string.h>
+
+int main(int argc, char **argv)
+{
+    if (argc != 3) {
+        printf("usage: chmod <octal-mode> <path>\n");
+        return 1;
+    }
+
+    mode_t mode;
+    int rc = sscanf(argv[1], "%o", &mode);
+    if (rc != 1) {
+        perror("sscanf");
+        return 1;
+    }
+
+    rc = chmod(argv[2], mode);
+    if (rc < 0) {
+        perror("chmod");
+        return 1;
+    }
+
+    return 0;
+}