Quote/unquote GRUB values only when saving.

Preserve the original (quoted) strings internally, but show them in a
user-friendly manner. Implemented to minimize changes made to internal
structures. Also useful for command substibutions using echo. For
example:

GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`

but the user doesn't see this ugly command, he only sees the result.