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/ls.cpp

@@ -152,6 +152,8 @@ int do_dir(const char* path)
 
         printf(" %10u  ", st.st_size);
 
+        printf(" %10u  ", st.st_mtime);
+
         print_name(st, de->d_name, pathbuf);
 
         printf("\n");