os-release: sanitize required fields

Currently only VERSION_ID field is sanitized, but os-release (5) has
more fields with the same requirement. Moreover, those fields come
unquoted in most distributions, because quotes are not needed for a
values without whitespaces.

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>