Implement symlinks

Fixes bradfitz/docker#15
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #24 from LK4D4/implement_symlink_#15

Implement symlink
Implement Utimens

Fixes bradfitz/docker#7
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)