add OpenBSD unveil support

add OpenBSD unveil support

The unveil() system call first appeared in OpenBSD 6.4.

For stagit it has the following properties now:

- stagit-index: only read-access to the file-system for the specified
  directories/repositories.
- stagit: read-access to the specified directory/repository.
  read-write and creation access to the current directory for the output files.
  read-write and creation access to the specified cache file.