MFC r356688, r356694: tap(4) manpage improvements

r356688:
Install tap(4) manpage as vmnet(4) as well

If one comes across a vmnet interface, this is a useful pointer to have
towards what it actually is if they're otherwise unfamiliar.

r356694:
tap(4): also note that we drop configured addresses

This provides a specific pointer for users of tap(4) to understand why their
interfaces are losing their addresses, and specifically how to workaround
this if they need different behavior.

This manpage received a .Dd bump earlier today in r35688, so no bump occurs
this time.