Generate a tinc-up script from an invitation.

Generate a tinc-up script from an invitation.

This adds the ability for an invitation to provision an invitee with a
tinc-up script. This is quite strictly controlled; only address configuration
and routes are supported by adding "Ifconfig" and "Route" statements to
the invitation file. The "tinc join" command will generate a tinc-up script
from those statements, and will ask before enabling the tinc-up script.