import-creds: unify acquire_credential_directory() + acquire_encrypted_credential_dir...

import-creds: unify acquire_credential_directory() + acquire_encrypted_credential_directory()

Let's unify these very similar functions, and port them to the new
mount_credentials_fs() call.

While we are at it, if we detect that the credentials dir already is a
mount point, remount it writable so that we can actually write to it.