src: create element command

src: create element command

This patch adds the create command, that send the NLM_F_EXCL flag so
nf_tables bails out if the element already exists, eg.

 # nft add element x y { 1.1.1.1 }
 # nft create element x y { 1.1.1.1 }
 <cmdline>:1:1-31: Error: Could not process rule: File exists
 create element x y { 1.1.1.1 }
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This update requires nf_tables kernel patches to honor the NLM_F_EXCL.

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