release: Made it possible to specify more attributes in metalog entries

Verify that the target file or directory exists, and add support for an
optional mode. This is required to enable NO_ROOT builds of some
cloudware image types. No functional change intended.

Reviewed by: cperciva, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52449