jail: add 'kill' method to container.%s object

jail: add 'kill' method to container.%s object

Using the the current container signal method to send a signal to the
jailed process works fine, as signals are being forwarded by the
ujail parent process. However, in case of KILL (==9) signal, both,
parent and jailed process are killed immediately which results in the
'poststop' OCI hook being skipped.
Add new 'kill' method to ujail's container object to allow sending
signals to the jailed process directly instead of having to send
signals to the parent.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>