refactor(libcontainer): rename to CapabilitiesMask

The Capabilities field on libcontainer is actually used as a mask.
Rename the field so that this is more clear.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
chore(libcontainer): small grammar fix in types_test

Someone probably got really used to typing er on the end of contain :)

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)