`utime` not defined

[jounathaen]

utime.h shall define utime(). We currently only define utimbuf.
However, I believe that Hermit does not have a utime implementation (?), so I wonder if it would be better to remove utime.h completely 🤔

1. Stub utime.
2. Optional: fake utime
3. Set file access and modification times kernel#1724
4. Implement utime