tools/bpftool: Remove libbpf_internal.h usage in bpftool

Most netlink-related functions were unique to bpftool usage, so I moved them
into net.c. Few functions are still used by both bpftool and libbpf itself
internally, so I've copy-pasted them (libbpf_nl_get_link,
libbpf_netlink_open). It's a bit of duplication of code, but better separation
of libbpf as a library with public API and bpftool, relying on unexposed
functions in libbpf.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200819013607.3607269-3-andriin@fb.com

tools/bpf/bpftool/gen.c

@@ -19,11 +19,9 @@
 #include <sys/mman.h>
 #include <bpf/btf.h>
 
-#include "bpf/libbpf_internal.h"
 #include "json_writer.h"
 #include "main.h"
 
-
 #define MAX_OBJ_NAME_LEN 64
 
 static void sanitize_identifier(char *name)