Add new IS_INTERACTIVE flag. This lets you specify that the port
is an interactive port, and requires user input somewhere along
the way (either fetching, configuring, building or installing).

If the user then sets BATCH in their environment, this port will be
skipped. If the user sets INTERACTIVE, then ONLY those ports marked
interactive are run (allowing one to do all ports in two passes).
If the user sets both BATCH and INTERACTIVE, then a metal claw extends
from the CRT and brutally yanks their nose off.