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-typing.go

@@ -31,7 +31,7 @@ type notReadOnly interface {
 	notReadOnly()
 }
 
-//// define types by adding methods
+// // define types by adding methods
 // hasPath
 func (p *sshFxpLstatPacket) getPath() string    { return p.Path }
 func (p *sshFxpStatPacket) getPath() string     { return p.Path }