ipvs: add ipv6 support to ftp

Add support for FTP commands with extended format (RFC 2428):

- FTP EPRT: IPv4 and IPv6, active mode, similar to PORT
- FTP EPSV: IPv4 and IPv6, passive mode, similar to PASV.
EPSV response usually contains only port but we allow real
server to provide different address

We restrict control and data connection to be from same
address family.

Allow the "(" and ")" to be optional in PASV response.

Also, add ipvsh argument to the pkt_in/pkt_out handlers to better
access the payload after transport header.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>