fix content-type detection in docker cp

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
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)
Merge pull request #4719 from philips/Capabilities-to-CapabilitiesMask

refactor(libcontainer): rename to CapabilitiesMask
Merge pull request #4720 from vieux/fix_content-type_cp

fix content-type detection in docker cp