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.

NOTE: This is completely untested, wanted to make sure @creack agreed
with me first.

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