xtables-translate: fix issue with quotes

xtables-translate: fix issue with quotes

Some translations included escaped quotes when they were called from
nft:

$ sudo nft list ruleset
table ip mangle {
    chain FORWARD {
        type filter hook forward priority -150; policy accept;
        ct helper \"ftp\" counter packets 0 bytes 0
                  ^^   ^^
    }
}

This behavior is only correct when xlate functions are called from a
xtables-translate command. This patch solves that issue using a new
parameter (escape_quotes) in the xlate functions.

Signed-off-by: Pablo M. Bermudo Garay <pablombg@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>