Partially fix interactive mode in patch

Partially fix interactive mode in patch

If ask() can't open /dev/tty for reading, it assumes the default answer
and carries on. Add missing unveil, so that ask() waits for an answer.
This isn't a full fix since it won't allow giving paths outside the tree
rooted at the current directory, but that's expected by the recent change.
Questions are only asked outside of force or batch mode.

fix suggested by op and semarie
ok deraadt florian