extensions: libxt_SYNPROXY: initial manual page

Signed-off-by: Martin Topholm <mph@one.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
libxtables: Print meaningful error message for an invalid MAC address string

If an invalid MAC address is used on the commmand line, the error message
`ether' was not really describing the problem.

Signed-off-by: Mart Frauenlob <mart.frauenlob@chello.at>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>