scanner: IPv4-Mapped IPv6 addresses support

scanner: IPv4-Mapped IPv6 addresses support

The scanner rejects IPv4-Mapped IPv6 addresses, eg.

 # cat test
 #!/usr/sbin/nft -f
 flush ruleset
 table inet global {
    set blackhole_ipv6 {
        type ipv6_addr
        flags interval
        elements = { ::ffff:0.0.0.0/96 }
    }
 }

 # nft -f test
 test:8:30-38: Error: syntax error, unexpected string, expecting comma or '}'
        elements = { ::ffff:0.0.0.0/96 }
                            ^^^^^^^^^^

According to RFC4291, Sect. 2.5.5.2. IPv4-Mapped IPv6 Address:

   |                80 bits               | 16 |      32 bits        |
   +--------------------------------------+--------------------------+
   |0000..............................0000|FFFF|    IPv4 address     |
   +--------------------------------------+----+---------------------+

Update scanner bits to parse this.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1188
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>