detect whether portsnap supports interactive-flag

I'm using puppet extensively and manage my poudriere based pkgng repositories with it (https://github.com/skoef/puppet-poudriere). Creating new port trees from puppet however is not possible since portsnap detects not being run from a terminal. With the new --interactive flag that has come with portsnap in FreeBSD 10.x, we can force portsnap to run interactively and therefore create port trees from non-interactive shells like cron or puppet.
This pull-request adds support for the portsnap --interactive flag to poudriere.