[Linux] os-release: unescape quotes / backslashes

The os-release specification states that single quotes, double quotes
and backslashes are supposed to be escaped with a backslash. Unfortunately
I missed this in my original commit 7a9aa6f5d9ac8df1c79eb06ed63bce8af162c3ba.

http://www.freedesktop.org/software/systemd/man/os-release.html