Set file access and modification times

[mkroening]

The kernel should provide some functionality to set file access and modification times. This would probably entail system calls corresponding to futimens, utimensat, utimes:

• int futimens(int fd, const struct timespec times[2]);
• int utimensat(int fd, const char *path, const struct timespec times[2], int flag);
• int utimes(const char *path, const struct timeval times[2]);