Split brain: With patches-unapplied trees, apply before running rules

Everywhere we invoke dpkg-buildpackage, check if we need to apply the
patches.  And at the end of every command that might have done this,
check if we did, and unapply them if so.

We don't try to unapply patches in case of error.  That seems likely
to be quite fragile.

Print a warning when we apply patches, suggesting ways to avoid the
need.