libnftables: export public symbols only

libnftables: export public symbols only

Export public symbols (the library API functions) instead of all symbols in
the library.

This patch introduces the required macros to manage the visibility attributes
(mostly copied from libnftnl.git) and also marks each symbol as exported when
they need to be public. Also, introduce a .map file for proper symbol
versioning.

Previous to this patch, libnftables public symbols were:

% dpkg-gensymbols -q -plibnftables -v0.9.1 -O -esrc/.libs/libnftables.so.1 | wc -l
527

With this patch, libnftables symbols are:

% dpkg-gensymbols -q -plibnftables -v0.9.1 -O -esrc/.libs/libnftables.so.1
libnftables.so.1 libnftables #MINVER#
 nft_ctx_add_include_path@Base 0.9.1
 nft_ctx_buffer_error@Base 0.9.1
 nft_ctx_buffer_output@Base 0.9.1
 nft_ctx_clear_include_paths@Base 0.9.1
 nft_ctx_free@Base 0.9.1
 nft_ctx_get_dry_run@Base 0.9.1
 nft_ctx_get_error_buffer@Base 0.9.1
 nft_ctx_get_output_buffer@Base 0.9.1
 nft_ctx_new@Base 0.9.1
 nft_ctx_output_get_debug@Base 0.9.1
 nft_ctx_output_get_flags@Base 0.9.1
 nft_ctx_output_set_debug@Base 0.9.1
 nft_ctx_output_set_flags@Base 0.9.1
 nft_ctx_set_dry_run@Base 0.9.1
 nft_ctx_set_error@Base 0.9.1
 nft_ctx_set_output@Base 0.9.1
 nft_ctx_unbuffer_error@Base 0.9.1
 nft_ctx_unbuffer_output@Base 0.9.1
 nft_run_cmd_from_buffer@Base 0.9.1
 nft_run_cmd_from_filename@Base 0.9.1

Signed-off-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>