include: fix possible clash in ifdef namespace

Use _LIBNFTABLES_ prefix to avoid possible clash with headers that
are defined in other libraries that may be used by third party
applications.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
src: consolidate netlink build header function

Add new function nft_nlmsg_build_hdr which consolidates all existing
functions to build headers per object. They basically look the same.
This patch still provides aliases for consistency in the naming
approach.

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