build: combine iptables-multi and iptables-static

build: combine iptables-multi and iptables-static

Changed the Makefile so that:

1. --enable-shared / --disable-shared control the linkage against
   libdl (and thus the potential to use 3rd party extensions)

2. --enable-static / --disable-static controls whether shipped
   extensions are built-in or provided as modules

iptables-static becomes redundant by this action; iptables-multi now
has the feature.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>