Add `gpg` compat to `bs`

[dhuseby]

The bs cli tool needs to support a --gpg command line option as well as look at argv[0] to see if the process was executed using a gpg alias to detect if it should run in gpg compatibility mode.

When in gpg compat mode, the command line options for gpg will work the same with bs. At a very minimum we need to support GPG's -bsau options and the --status-fd option. That's how git interfaces with gpg. This will make bs a drop-in replacement for gpg when using it with git.