Support quoting strings with " or ' in an iproute2 batch file.

Enables to configure empty crypto keys (for ESP-null) or keys with
spaces:

    xfrm state add src 1.1.1.1 dst 2.2.2.2 proto ah spi 0x1 \
        mode tunnel auth hmac(sha1) "r4ezR/@kd6'749f2 6zf$"

    xfrm state add src 5.5.5.5 dst 2.2.2.2 proto esp spi 0x2 \
        mode tunnel enc cipher_null ""
Signed-off-by: Christophe Gouault <christophe.gouault@6wind.com>