src: add create set command

src: add create set command

Add support for the 'create' command, we already support this in other
existing objects, so support this for sets too, eg.

 # nft add set x y { type ipv4_addr\; }
 # nft create set x y { type ipv4_addr\; }
 <cmdline>:1:1-35: Error: Could not process rule: File exists
 create set x y { type ipv4_addr; }
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 # nft add set x y { type ipv4_addr\; }
 #

This command sets the NLM_F_EXCL netlink flag, so if the object already
exists, nf_tables returns -EEXIST.

This is changing the existing behaviour of 'nft add set' which was
setting this flag, this is inconsistent with regards to the way other
objects behave.

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