Shutting down (and optionally restarting) multiple interfaces is
fraught with problems if there are loops in the interface connectivity
(e.g. the HTTP content-decoded and transfer-decoded interfaces, which
will generally loop back to each other).  Various workarounds
currently exist across the codebase, generally involving preceding
calls to intf_nullify() to avoid problems due to known loops.

Provide intfs_shutdown() and intfs_restart() to allow all of an
object's interfaces to be shut down (or restarted) in a single call,
without having to worry about potential external loops.

Signed-off-by: Michael Brown <mcb30@ipxe.org>