cleanup open_tun() for TARGET_NETBSD

cleanup open_tun() for TARGET_NETBSD

- NetBSD "dynamic tap" (--dev tap -> tap<number>) handling had special
  #ifdef'ed code inside open_tun_generic() - pull out, move to NetBSD
  open_tun().  Roughly the same amount of code, less #ifdef, code flow
  is more clear.

- fix one spurious warning about "remote" not being initialized

- adjust NetBSD do_open() comments to actual code - the "pre NetBSD 4.0"
  code has long be removed, but the comment was still there.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20220808152344.17539-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24849.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>