rework criu check logic

The "criu check" command to check if the kernel is properly configured
to run criu is broken.

The "criu check --ms" command used to be the way to tell criu to check
only for features that have been merged upstream.  But recent kernels
have a set of features whose presence doesn't necessarily mean that
dump or restore will fail but rather *may* fail depending on whether
the process tree uses those features.

This patch deprecates --ms and introduces --extra, --experimental,
and --all.  See "criu --help" or "man criu" for more info.

Typical use cases are:

	$ sudo criu check
	<zero or more warnings and errors...>
	Looks good.
	$ echo $?
	0

	$ sudo criu check --extra
	<zero or more warnings and errors...>
	Looks good.
	$ echo $?
	1

	$ sudo criu check --extra
	<one or more warnings...>
	Looks good but some kernel features are missing
	which, depending on your process tree, may cause
	dump or restore failure.
	$ echo $?
	1

	$ sudo criu check --feature list
	mnt_id aio_remap timerfd tun userns fdinfo_lock seccomp_suspend \
		seccomp_filters loginuid cgroupns

	$ sudo criu check --feature mnt_id
	Warn  (cr-check.c:283): fdinfo doesn't contain the mnt_id field
	$ echo $?
	1

	$ sudo criu check --feature tun
	tun is supported
	$ echo $?
	0

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>