interface: allow renaming interface when moving to jail netns

interface: allow renaming interface when moving to jail netns

Introduce jail_ifname option to define the name of a Linux network
interface when moved into a jail's network namespace.
This is useful for containers which expect the network interface to
have a specific name (eg. 'host0' in case of systemd).
While at it, clean-up and fix bugs in jail interface up/down routines.

Signed-off-by: Daniel Golle <[email protected]>