support multi-image (docker) archives

Support loading and saving tarballs with more than one image.
Add a new `/libpod/images/export` endpoint to the rest API to
allow for exporting/saving multiple images into an archive.

Note that a non-release version of containers/image is vendored.
A release version must be vendored before cutting a new Podman
release.  We force the containers/image version via a replace in
the go.mod file; this way go won't try to match the versions.

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

vendor/github.com/mattn/go-isatty/isatty_tcgets.go

@@ -1,18 +0,0 @@
-// +build linux aix
-// +build !appengine
-
-package isatty
-
-import "golang.org/x/sys/unix"
-
-// IsTerminal return true if the file descriptor is terminal.
-func IsTerminal(fd uintptr) bool {
-	_, err := unix.IoctlGetTermios(int(fd), unix.TCGETS)
-	return err == nil
-}
-
-// IsCygwinTerminal return true if the file descriptor is a cygwin or msys2
-// terminal. This is also always false on this environment.
-func IsCygwinTerminal(fd uintptr) bool {
-	return false
-}