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>

CHANGELOG.md

@@ -6,6 +6,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Deprecated
+
+ * `runc` option `--criu` is now ignored (with a warning), and the option will
+   be removed entirely in a future release. Users who need a non-standard
+   `criu` binary should rely on the standard way of looking up binaries in
+   `$PATH`. (#3316)
+
 ### Changed
 
  * When Intel RDT feature is not available, its initialization is skipped,