Wait for interface existence?

I find myself wanting a tool that waits until a certain set of network
interfaces exists.  (I want this so I can, for example, have a
boot-time script wait until tap0 and tap1 both exist, then do whatever
is appropriate.)

I've implemented it in ifconfig in my 5.2 derivative, in the form of
"ifconfig -W name name ...".  But, on checking cvsweb, I find -W
already means something different in -current.

Is there any interest in NetBSD adopting this functionality as an
ifconfig flag?  If so, I'll change my option to match NetBSD's (no
point in gratuitous incompatibilities); if not, I'll probably stick
with -W because whatever I pick will be at risk of being taken for some
other purpose in future NetBSD.

I can, of course, offer my implementation, though I don't know whether
ifconfig has drifted far enough that it wouldn't be useful (either
verbatim or as a starting point).

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