Fix cap drop issues with lxc

This uses "," instead of spaces so that the flags are parsed correctly
and also does not do a strings.Split on an empty string because
strings.Split will return a slice with one element, and empty string
causing parsing to fail when it validates that the cap exists.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Use : to split caps in sysinit flags
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #7062 from crosbymichael/fix-lxc-caps

Fix cap drop issues with lxc