runc exec: reject paused container unless --ignore-paused

Currently, if a container is paused (i.e. its cgroup is frozen), runc exec
just hangs, and it is not obvious why.

Refuse to exec in a paused container. Add a test case.

In case runc exec in a paused container is a legit use case,
add --ignore-paused option to override the check. Document it,
add a test case.

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

contrib/completions/bash/runc

@@ -173,6 +173,7 @@ _runc_exec() {
 	   --apparmor
 	   --cap, -c
 	   --preserve-fds
+	   --ignore-paused
 	"
 
 	local all_options="$options_with_args $boolean_options"