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/request-plan9.go

@@ -1,10 +1,9 @@
+//go:build plan9
 // +build plan9
 
 package sftp
 
 import (
-	"path"
-	"path/filepath"
 	"syscall"
 )
 
@@ -15,20 +14,3 @@ func fakeFileInfoSys() interface{} {
 func testOsSys(sys interface{}) error {
 	return nil
 }
-
-func toLocalPath(p string) string {
-	lp := filepath.FromSlash(p)
-
-	if path.IsAbs(p) {
-		tmp := lp[1:]
-
-		if filepath.IsAbs(tmp) {
-			// If the FromSlash without any starting slashes is absolute,
-			// then we have a filepath encoded with a prefix '/'.
-			// e.g. "/#s/boot" to "#s/boot"
-			return tmp
-		}
-	}
-
-	return lp
-}