Saturate negative memory stat values at '0'.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #201 from vishh/stats_rounding

Saturate negative memory stat values at '0'.
Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>