iptables: restore NOTRACK functionality, target aliasing

iptables: restore NOTRACK functionality, target aliasing

Commit v1.4.16-1-g2aaa7ec is testing for real_name (not) being NULL
which was always false (true). real_name was never NULL, so cs->jumpto
would always be used, which rendered -j NOTRACK unusable, since the
chosen real name.revision is for example NOTRACK.1, which does not exist
at the kernel side.

	# ./iptables/xtables-multi main4 -t raw -A foo -j NOTRACK
	dbg: Using NOTRACK.1
	WARNING: The NOTRACK target is obsolete. Use CT instead.
	iptables: Protocol wrong type for socket.

To reasonably support the extra-special verdict names, make it so that
real_name remains NULL when an extension defined no alias, which we can
then use to determine whether the user entered an alias name (which
needs to be followed) or not.

[ I have mangled this patch to remove a comment unnecessarily large.
  BTW, this patch gets this very close to the initial target aliasing
  proposal --pablo ]

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>