tun(4)/tap(4): allow devices to be configured as transient

Transient tunnel devices are removed immediately after last close, so
that an application that's created the tunnel could eliminate the nee

tun(4)/tap(4): allow devices to be configured as transient

Transient tunnel devices are removed immediately after last close, so
that an application that's created the tunnel could eliminate the need
to manually destroy the tunnel whose lifetime it's already managing.

Reviewed by: zlei
Differential Revision: https://reviews.freebsd.org/D44200

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