add --module flag

Support a new concept in containers.conf called "modules".  A "module"
is a containers.conf file located at a specific directory.  More than
one module can be loaded in the specified order, following existing
override semantics.

There are three directories to load modules from:
 - $CONFIG_HOME/containers/containers.conf.modules
 - /etc/containers/containers.conf.modules
 - /usr/share/containers/containers.conf.modules

With CONFIG_HOME pointing to $HOME/.config or, if set, $XDG_CONFIG_HOME.
Absolute paths will be loaded as is, relative paths will be resolved
relative to the three directories above allowing for admin configs
(/etc/) to override system configs (/usr/share/) and user configs
($CONFIG_HOME) to override admin configs.

Pulls in containers/common/pull/1599.

Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

vendor/github.com/pkg/sftp/packet-manager.go

@@ -40,7 +40,7 @@ func newPktMgr(sender packetSender) *packetManager {
 	return s
 }
 
-//// packet ordering
+// // packet ordering
 func (s *packetManager) newOrderID() uint32 {
 	s.packetCount++
 	return s.packetCount
@@ -89,7 +89,7 @@ func (o orderedPackets) Sort() {
 	})
 }
 
-//// packet registry
+// // packet registry
 // register incoming packets to be handled
 func (s *packetManager) incomingPacket(pkt orderedRequest) {
 	s.working.Add(1)