os-release: Quote variables as documented in the manual

Variables must be quoted if they contain non-alphanumeric characters.

Warner noted in the review that the lack of quoting causing problems
here is rather an edge case. I believe that it's worth adding the quotes
here anyway because this is what the specification says and there is no
good reason not to follow it.

Reviewed by: imp
Approved by: imp (src)
MFC after: 7 days