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-interfaces.go

@@ -74,6 +74,11 @@ type StatVFSFileCmder interface {
 // FileLister should return an object that fulfils the ListerAt interface
 // Note in cases of an error, the error text will be sent to the client.
 // Called for Methods: List, Stat, Readlink
+//
+// Since Filelist returns an os.FileInfo, this can make it non-ideal for implementing Readlink.
+// This is because the Name receiver method defined by that interface defines that it should only return the base name.
+// However, Readlink is required to be capable of returning essentially any arbitrary valid path relative or absolute.
+// In order to implement this more expressive requirement, implement [ReadlinkFileLister] which will then be used instead.
 type FileLister interface {
 	Filelist(*Request) (ListerAt, error)
 }
@@ -87,12 +92,33 @@ type LstatFileLister interface {
 }
 
 // RealPathFileLister is a FileLister that implements the Realpath method.
-// We use "/" as start directory for relative paths, implementing this
-// interface you can customize the start directory.
+// The built-in RealPath implementation does not resolve symbolic links.
+// By implementing this interface you can customize the returned path
+// and, for example, resolve symbolinc links if needed for your use case.
 // You have to return an absolute POSIX path.
 //
-// Deprecated: if you want to set a start directory use WithStartDirectory RequestServerOption instead.
+// Up to v1.13.5 the signature for the RealPath method was:
+//
+// # RealPath(string) string
+//
+// we have added a legacyRealPathFileLister that implements the old method
+// to ensure that your code does not break.
+// You should use the new method signature to avoid future issues
 type RealPathFileLister interface {
+	FileLister
+	RealPath(string) (string, error)
+}
+
+// ReadlinkFileLister is a FileLister that implements the Readlink method.
+// By implementing the Readlink method, it is possible to return any arbitrary valid path relative or absolute.
+// This allows giving a better response than via the default FileLister (which is limited to os.FileInfo, whose Name method should only return the base name of a file)
+type ReadlinkFileLister interface {
+	FileLister
+	Readlink(string) (string, error)
+}
+
+// This interface is here for backward compatibility only
+type legacyRealPathFileLister interface {
 	FileLister
 	RealPath(string) string
 }
@@ -105,11 +131,19 @@ type NameLookupFileLister interface {
 	LookupGroupName(string) string
 }
 
-// ListerAt does for file lists what io.ReaderAt does for files.
-// ListAt should return the number of entries copied and an io.EOF
-// error if at end of list. This is testable by comparing how many you
-// copied to how many could be copied (eg. n < len(ls) below).
+// ListerAt does for file lists what io.ReaderAt does for files, i.e. a []os.FileInfo buffer is passed to the ListAt function
+// and the entries that are populated in the buffer will be passed to the client.
+//
+// ListAt should return the number of entries copied and an io.EOF error if at end of list.
+// This is testable by comparing how many you copied to how many could be copied (eg. n < len(ls) below).
 // The copy() builtin is best for the copying.
+//
+// Uid and gid information will on unix systems be retrieved from [os.FileInfo.Sys]
+// if this function returns a [syscall.Stat_t] when called on a populated entry.
+// Alternatively, if the entry implements [FileInfoUidGid], it will be used for uid and gid information.
+//
+// If a populated entry implements [FileInfoExtendedData], extended attributes will also be returned to the client.
+//
 // Note in cases of an error, the error text will be sent to the client.
 type ListerAt interface {
 	ListAt([]os.FileInfo, int64) (int, error)