Enforce sync_enc option

* src/main.c (read_options): Always enable sync_enc.

This prevents the situation that we save unencrypted contents
in MAPI. Something the branch oom_attachments tries to archive
in an async way. For now to have a mitigation for the fact
that unencrypted drafts are synced to the server even when
draft encryption is on.

• GnuPG-Bug-Id: T5022