Add a way to detect that encryption is not supported

[ondrejholy]

The password is requested by gnome-autoar, when archive_entry_is_encrypted returns TRUE. This is fine for ZIP, but not for RAR/7ZIP, or perhaps some other formats, where the subsequent archive_read_data_block (or archive_read_next_header in the case of archive_entry_is_metadata_encrypted) returns an error that it is not supported. I would suggest returning FALSE from archive_entry_is_encrypted resp. archive_entry_is_metadata_encrypted when the encryption is not supported or adding some new API to determine this situation before asking a user for a password. Or is there already a way to achieve this somehow?

Related to https://gitlab.gnome.org/GNOME/gnome-autoar/-/merge_requests/34 and #1374.