Add setting for format of CSRs

This makes it possible to specify (with a config key) whether CSRs
shall be saved in binary format or as ASCII-armored PEM. By default,
CSRs are saved in binary format again because some CAs require this
format. This default reverts a change that was made with commit
fea96ba9999effbe7e29ff1729aaf3076c0d0b65
where the format was changed from binary to PEM (to unify it with CSRs
generated for card keys).

• GnuPG-bug-id: T8115