drop CAP_SYSLOG capability

Kernel capabilities for privileged syslog operations are currently splitted into
CAP_SYS_ADMIN and CAP_SYSLOG since the following commit:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ce6ada35bdf710d16582cc4869c26722547e6f11

This patch drops CAP_SYSLOG to prevent containers from messing with
host's syslog (e.g. `dmesg -c` clears up host's printk ring buffer).

Closes #5491

Docker-DCO-1.1-Signed-off-by: Eiichi Tsukata <devel@etsukata.com> (github: Etsukata)
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

daemon/execdriver/lxc/init.go

@@ -149,6 +149,7 @@ func setupCapabilities(args *execdriver.InitArgs) error {
 		capability.CAP_MAC_OVERRIDE,
 		capability.CAP_MAC_ADMIN,
 		capability.CAP_NET_ADMIN,
+		capability.CAP_SYSLOG,
 	}
 
 	c, err := capability.NewPid(os.Getpid())

daemon/execdriver/native/template/default_template.go

@@ -25,6 +25,7 @@ func New() *libcontainer.Container {
 			libcontainer.GetCapability("MAC_ADMIN"),
 			libcontainer.GetCapability("NET_ADMIN"),
 			libcontainer.GetCapability("MKNOD"),
+			libcontainer.GetCapability("SYSLOG"),
 		},
 		Namespaces: libcontainer.Namespaces{
 			libcontainer.GetNamespace("NEWNS"),