Avoid running /bin/ip which doesn't exist on FreeBSD.

I get the following error every time I run pptp:

/bin/ip: not found
/bin/ip: not found

Since /bin/ip doesn't exist on FreeBSD it doesn't make sense to try
and run it, and pptp appears to function without it (this change
certainly won't make things any worse). I've done a minimally
invasive change which just makes it run /usr/bin/true instead.