MINOR: log: support true cbor binary encoding

CBOR in hex format as implemented in previous commit is convenient because
the produced output is portable and can easily be embedded in regular
syslog payloads.

However, one of the goal of CBOR implementation is to be able to produce
"Concise Binary" object representation. Here is an excerpt from cbor.io
website:

  "Some applications also benefit from CBOR itself being encoded in
   binary. This saves bulk and allows faster processing."

Currently we don't offer that with '+cbor', quite the opposite actually
since a text string encoded with '+cbor' option will be larger than a
text string encoded with '+json' or without encoding at all, because for
each CBOR binary byte, 2 characters will be emitted.

Hopefully, the sink/log API allows for binary data to be passed as
parameter, this is because all relevant functions in the chain don't rely
on the terminating NULL byte and take a string pointer + string length as
parameter. We can actually rely on this property to support the '+bin'
option when combined with '+cbor' to produce RAW binary CBOR output.
Be careful though, as this is only intended for use with set-var-fmt or to
send binary data to capable UDP/ring endpoints.

Example:
  log-format "%{+cbor,+bin}o %(test)[bin(00AABB)]"

Will produce:
  bf64746573745f4300aabbffff

(output was piped to `hexdump  -ve '1/1 "%.2x"'` to dump raw bytes as HEX
characters)

With cbor.me pretty printer, it gives us:
  BF              # map(*)
     64           # text(4)
        74657374  # "test"
     5F           # bytes(*)
        43        # bytes(3)
           00AABB # "\u0000\xAA\xBB"
        FF        # primitive(*)
     FF           # primitive(*)

doc/configuration.txt

@@ -25759,6 +25759,10 @@ Flags are :
           syslog endpoints.
           As with json encoding, incomplete numerical values will be encoded
           as-is and '+E' option will be ignored.
+          When combined with '+bin' option, it will directly generate raw
+          binary CBOR payload. Be careful, because it will obviously generate
+          non-printable chars, thus it is mainly intented for use with
+          set-var-fmt, rings and binary-capable log endpoints.
 
   Example:
 

src/log.c

@@ -1774,8 +1774,15 @@ static char *_encode_byte_hex(char *start, char *stop, unsigned char byte)
 static char *_lf_cbor_encode_byte(struct cbor_encode_ctx *cbor_ctx,
                                   char *start, char *stop, unsigned char byte)
 {
-	__maybe_unused struct lf_buildctx *ctx = cbor_ctx->e_byte_fct_ctx;
+	struct lf_buildctx *ctx = cbor_ctx->e_byte_fct_ctx;
 
+	if (ctx->options & LOG_OPT_BIN) {
+		/* raw output */
+		if ((stop - start) < 1)
+			return NULL;
+		*start++ = byte;
+		return start;
+	}
 	return _encode_byte_hex(start, stop, byte);
 }