Option -a for runc kill is only usable when we are trying to kill -9 the container which does not have its own PID namespace. All other use cases (like sending SIGTERM to all container processes) are questionable to say at least.

I propose we deprecate -a, and handle the above use case (sending SIGKILL to init of the container which does not have its own pidns) automatically (in fact, this is already done in libcontainer, but not in runc binary).

The deprecation can be done in steps:

1. In runc 1.2, emit a warning when -a is used with runc kill.
2. In runc 1.3 (or later version), upgrade the warning to error.
3. In runc 1.4, drop the flag.