Implement utime() along with a naive /bin/touch.

This synchronous approach to inodes is silly, obviously. I need to rework
it so that the in-memory CoreInode object is the canonical inode, and then
we just need a sync() that flushes pending changes to disk.

Userland/touch.cpp

@@ -0,0 +1,16 @@
+#include <stdio.h>
+#include <utime.h>
+#include <sys/types.h>
+
+int main(int argc, char** argv)
+{
+    if (argc != 2) {
+        fprintf(stderr, "usage: touch <path>\n");
+        return 1;
+    }
+    int rc = utime(argv[1], nullptr);
+    if (rc < 0)
+        perror("utime");
+    return 0;
+}
+