bpftool: add ability to dump BTF types

Add new `btf dump` sub-command to bpftool. It allows to dump
human-readable low-level BTF types representation of BTF types. BTF can
be retrieved from few different sources:
  - from BTF object by ID;
  - from PROG, if it has associated BTF;
  - from MAP, if it has associated BTF data; it's possible to narrow
    down types to either key type, value type, both, or all BTF types;
  - from ELF file (.BTF section).

Output format mostly follows BPF verifier log format with few notable
exceptions:
  - all the type/field/param/etc names are enclosed in single quotes to
    allow easier grepping and to stand out a little bit more;
  - FUNC_PROTO output follows STRUCT/UNION/ENUM format of having one
    line per each argument; this is more uniform and allows easy
    grepping, as opposed to succinct, but inconvenient format that BPF
    verifier log is using.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/main.c

@@ -56,7 +56,7 @@ static int do_help(int argc, char **argv)
 		"       %s batch file FILE\n"
 		"       %s version\n"
 		"\n"
-		"       OBJECT := { prog | map | cgroup | perf | net | feature }\n"
+		"       OBJECT := { prog | map | cgroup | perf | net | feature | btf }\n"
 		"       " HELP_SPEC_OPTIONS "\n"
 		"",
 		bin_name, bin_name, bin_name);
@@ -188,6 +188,7 @@ static const struct cmd cmds[] = {
 	{ "perf",	do_perf },
 	{ "net",	do_net },
 	{ "feature",	do_feature },
+	{ "btf",	do_btf },
 	{ "version",	do_version },
 	{ 0 }
 };

tools/bpf/bpftool/main.h

@@ -150,6 +150,7 @@ int do_perf(int argc, char **arg);
 int do_net(int argc, char **arg);
 int do_tracelog(int argc, char **arg);
 int do_feature(int argc, char **argv);
+int do_btf(int argc, char **argv);
 
 int parse_u32_arg(int *argc, char ***argv, __u32 *val, const char *what);
 int prog_parse_fd(int *argc, char ***argv);