Represent Linux capabilites as an enum

This avoids errors when entering capabilities due to different capabilty
names, e.g. "MKNOD" in docker vs. "CAP_MKNOD" in Linux manpages.
It also gives a natural home for documenting the meaning of the
individual values.
Allow to manage Linux capabilities in CreateContainerCmd

This was previously only availabale in StartContainerCmd.
The introduction of HostConfig as a parameter to /containers/create in
Docker API v1.15 allows to specify capabilites here as well.
Merge pull request #106 from albers/capabilities

Allow to manage Linux capabilities in CreateContainerCmd