Make ifconfig ifN -vnet <jname|jid> actually work:
Make ifconfig ifN -vnet <jname|jid> actually work:

- fix ifconfig to ignore the non-existent interface in the current
  network stack in case of '-vnet'.
- in ifconfig: actually use the local variables defined for the
  vnet functions rather than modifying the global.

Reviewed by:	rwatson
Approved by:	re (kib)

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