Allow an IPsec tunnel interface to be renumbered

Allow an IPsec tunnel interface to be renumbered

When creating an IPsec tunnel interface, allow a numeric
identifier to be set for use in the interface's name in
place of the dev instance. Default to using the dev instance
if no value is explicitly set.

When an IPsec tunnel is deleted, the interface is deleted
now instead of being kept in a pool of available hw
interfaces. Otherwise there was the possibility of
conflicting tx node names between deleted tunnels and
newly created ones.

Change-Id: Ic525466622a0dec38a845fa5871c084f6d9da380
Signed-off-by: Matthew Smith <mgsmith@netgate.com>