libnftables: Split code into frontend and library

libnftables: Split code into frontend and library

This finally creates the libnftables shared object.

For some reason, this causes two compiler warnings to appear:

| parser_bison.y: In function 'nft_parse':
| parser_bison.y:131:3: warning: implicit declaration of function 'nft_set_debug' [-Wimplicit-function-declaration]
|    nft_set_debug(1, scanner);
|    ^~~~~~~~~~~~~
| parser_bison.c:64:25: warning: implicit declaration of function 'nft_lex' [-Wimplicit-function-declaration]
|  #define yylex           nft_lex
|                          ^
| parser_bison.c:4745:16: note: in expansion of macro 'yylex'
|        yychar = yylex (&yylval, &yylloc, scanner);

So this patch contains a workaround, namely declaring both functions
in src/parser_bison.y. During linking the objects are found, so this is
rather a matter of cosmetics.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>