scripts/config: properly report and set string options

Currently, scripts/config removes the leading double-quote from
string options, but leaves the trailing double-quote.

Also, double-quotes in a string are escaped, but scripts/config
does not unescape those when printing

Finally, scripts/config does not escape double-quotes when setting
string options.

Eg. the current behavior:
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Bar \"Buz\" Meh"
    $ ./scripts/config -s FOO
    Bar \"Buz\" Meh"
    $ ./scripts/config --set-str FOO 'Alpha "Bravo" Charlie'
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Alpha "Bravo" Charlie"

Fix those three, giving this new behavior:
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Bar \"Buz\" Meh"
    $ ./scripts/config -s FOO
    Bar "Buz" Meh
    $ ./scripts/config --set-str FOO 'Alpha "Bravo" Charlie'
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Alpha \"Bravo\" Charlie"

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>