allow lxc.cap.keep = none

allow lxc.cap.keep = none

Commit 1fb86a7c introduced a way to drop capabilities without having to
specify them all explicitly. Unfortunately, there is no way to drop them
all, as just specifying an empty keep list, ie:

    lxc.cap.keep =

clears the keep list, causing no capabilities to be dropped.

This change allows a special value "none" to be given, which will clear
all keep capabilities parsed up to this point. If the last parsed value
is none, all capabilities will be dropped.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>