src: add CMD_OBJ_SETELEMS

src: add CMD_OBJ_SETELEMS

This new command type results from expanding the set definition in two
commands: One to add the set and another to add the elements. This
results in 1:1 mapping between the command object to the netlink API.
The command is then translated into a netlink message which gets a
unique sequence number. This sequence number allows to correlate the
netlink extended error reporting with the corresponding command.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>