In iptunnel, declare loop variables inside the loop as done in
ip6tunnel.

Fix and simplify goto logic in ip6tunnel:
- Failure to read over header lines would have left fp opened.
- By returning directly upon fopen() failure, fp can be closed
  unconditionally in the end.

Use the same goto logic in iptunnel, as well.
Signed-off-by: Phil Sutter <phil@nwl.cc>