MINOR: net_helper: prepare the ip.fp() converter to support more options

It can make sense to support extra components in the fingerprint to ease
configuration, so let's change the 0/1 value to a bit field. We also turn
the current 1 (TCP options list) to 2 so that we'll reuse 1 for the TTL.

doc/configuration.txt

@@ -21137,10 +21137,14 @@ ip.fp([<mode>])
     - byte 3..4: tcp.win
     - byte 5..6: tcp.options.mss, or zero if absent
 
-  When the <mode> argument is not set or is zero, the fingerprint is solely
-  made of the 7 bytes described above. When the <mode> is 1, it starts by the
-  7-byte block above, and is followed by the list of TCP option kinds, for 0
-  to 40 extra bytes, as returned by "tcp.options_list".
+  The <mode> argument permits to append more information to the fingerprint. By
+  default, when the <mode> argument is not set or is zero, the fingerprint is
+  solely made of the 7 bytes described above. If <mode> is specified as another
+  value, it then corresponds to the sum of the following values, and the
+  respective components will be concatenated to the fingerprint, in the order
+  below:
+    -   2: the list of TCP option kinds, as returned by "tcp.options_list",
+           made of 0 to 40 extra bytes, is appended to the fingerprint
 
   Example:
 

src/net_helper.c

@@ -776,7 +776,7 @@ static int sample_conv_ip_fp(const struct arg *arg_p, struct sample *smp, void *
 			break;
 
 		/* option is complete, take a copy of it */
-		if (mode > 0)
+		if (mode & 2) // mode & 2: append tcp.options_list
 			trash->area[trash->data++] = smp->data.u.str.area[ofs];
 
 		if (smp->data.u.str.area[ofs] == 2 /* MSS */) {