Make libcontainer's CapabilitiesMask into a []string (Capabilities).

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
add support for CAP_FOWNER

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #5863 from vieux/pr5862

Make libcontainer's CapabilitiesMask into a []string (Capabilities). + 	add support for CAP_FOWNER