Make NetworkStats a pointer to help the callers ignore that stats if it cannot be returned for any reason.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Do not mark any network stats as 'omitempty' since the value '0' is legal for those stats.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #119 from vishh/fix_network_stats

Minor fixes to network stats
netlink_linux: add remove address call, refactor add address call to
share code with remove address call.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Tweaked makefile to always run tests on 'make all'

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
netlink/netlink_linux_test.go: tests for netlink add/rem ip

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)