runc: remove --criu option

This was introduced in an initial commit, back in the day when criu was
a highly experimental thing. Today it's not; most users who need it have
it packaged by their distro vendor.

The usual way to run a binary is to look it up in directories listed in
$PATH. This is flexible enough and allows for multiple scenarios (custom
binaries, extra binaries, etc.). This is the way criu should be run.

Make --criu a hidden option (thus removing it from help). Remove the
option from man pages, integration tests, etc. Remove all traces of
CriuPath from data structures.

Add a warning that --criu is ignored and will be removed.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

contrib/completions/bash/runc

@@ -231,12 +231,11 @@ _runc_runc() {
 		--log
 		--log-format
 		--root
-		--criu
 		--rootless
 	"
 
 	case "$prev" in
-	--log | --root | --criu)
+	--log | --root)
 		case "$cur" in
 		*:*) ;; # TODO somehow do _filedir for stuff inside the image, if it's already specified (which is also somewhat difficult to determine)
 		'')