tools: bpftool: change time format for program 'loaded at:' information

To make eBPF program load time easier to parse from "bpftool prog"
output for machines, change the time format used by the program. The
format now differs for plain and JSON version:

- Plain version uses a string formatted according to ISO 8601.
- JSON uses the number of seconds since the Epoch, wich is less friendly
  for humans but even easier to process.

Example output:

    # ./bpftool prog
    41298: xdp  tag a04f5eef06a7f555 dev foo
            loaded_at 2018-04-18T17:19:47+0100  uid 0
            xlated 16B  not jited  memlock 4096B

    # ./bpftool prog -p
    [{
            "id": 41298,
            "type": "xdp",
            "tag": "a04f5eef06a7f555",
            "gpl_compatible": false,
            "dev": {
                "ifindex": 14,
                "ns_dev": 3,
                "ns_inode": 4026531993,
                "ifname": "foo"
            },
            "loaded_at": 1524068387,
            "uid": 0,
            "bytes_xlated": 16,
            "jited": false,
            "bytes_memlock": 4096
        }
    ]

Previously, "Apr 18/17:19" would be used at both places.
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/prog.c

@@ -96,7 +96,10 @@ static void print_boot_time(__u64 nsecs, char *buf, unsigned int size)
 		return;
 	}
 
-	strftime(buf, size, "%b %d/%H:%M", &load_tm);
+	if (json_output)
+		strftime(buf, size, "%s", &load_tm);
+	else
+		strftime(buf, size, "%FT%T%z", &load_tm);
 }
 
 static int prog_fd_by_tag(unsigned char *tag)
@@ -245,7 +248,8 @@ static void print_prog_json(struct bpf_prog_info *info, int fd)
 		print_boot_time(info->load_time, buf, sizeof(buf));
 
 		/* Piggy back on load_time, since 0 uid is a valid one */
-		jsonw_string_field(json_wtr, "loaded_at", buf);
+		jsonw_name(json_wtr, "loaded_at");
+		jsonw_printf(json_wtr, "%s", buf);
 		jsonw_uint_field(json_wtr, "uid", info->created_by_uid);
 	}