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/containers/common/pkg/config/default_windows.go

@@ -2,19 +2,6 @@ package config
 
 import "os"
 
-// getDefaultImage returns the default machine image stream
-// On Windows this refers to the Fedora major release number
-func getDefaultMachineImage() string {
-	return "testing"
-}
-
-// getDefaultMachineUser returns the user to use for rootless podman
-// This is only for the hyperv and qemu implementations.  WSL's user
-// will be hardcoded in podman to "user"
-func getDefaultMachineUser() string {
-	return "core"
-}
-
 // isCgroup2UnifiedMode returns whether we are running in cgroup2 mode.
 func isCgroup2UnifiedMode() (isUnified bool, isUnifiedErr error) {
 	return false, nil