proc-cmdline: allow backslash escapes when parsing /proc/cmdline
    
    So far when parsing /proc/cmdline we'd consider backslashes as
    mechanisms for escaping whitepace or quotes. This changes things so that
    they are retained as they are instead. The kernel itself doesn't allow such
    escaping, and hence we shouldn't do so either (see lib/cmdline.c in the
    kernel sources; it does support "" quotes btw).
    
    This fix is useful to allow specifying backslash escapes in the "root="
    cmdline option to be passed through to systemd-fstab-generator. Example:
    
        root=/dev/disk/by-partlabel/Root\x20Partition
    
    Previously we'd eat up the "\" so that we'd then look for a device
    /dev/disk/by-partlabel/Rootx20Partition which never shows up.
    
    (cherry picked from commit d997861ea7dae633174cd80ab55552c020526b62)