MINOR: payload: allow the payload sample fetches to retrieve arbitrary lengths

When using req.payload and res.payload to look up for specific content at an
arbitrary location, we're often facing the problem of not knowing the input
buffer length. If the length argument is larger than the buffer length, the
function did not match, and if they're smaller, there is a risk of not getting
the expected content. This is especially true when looking for data in SOAP
requests.

So let's make some provisions for scanning the whole buffer by specifying a
length of 0 bytes. This greatly simplifies the processing of random-sized
input data.
diff --git a/src/payload.c b/src/payload.c
index 05418ac..227379b 100644
--- a/src/payload.c
+++ b/src/payload.c
@@ -589,7 +589,7 @@
 	if (!chn)
 		return 0;
 
-	if (!buf_size || buf_size > chn->buf->size || buf_offset + buf_size > chn->buf->size) {
+	if (buf_size > chn->buf->size || buf_offset + buf_size > chn->buf->size) {
 		/* will never match */
 		smp->flags = 0;
 		return 0;
@@ -600,8 +600,11 @@
 
 	/* init chunk as read only */
 	smp->type = SMP_T_CBIN;
-	chunk_initlen(&smp->data.str, chn->buf->p + buf_offset, 0, buf_size);
+	chunk_initlen(&smp->data.str, chn->buf->p + buf_offset, 0, buf_size ? buf_size : (chn->buf->i - buf_offset));
 	smp->flags = SMP_F_VOLATILE;
+	if (!buf_size && !channel_full(chn) && !channel_input_closed(chn))
+		smp->flags |= SMP_F_MAY_CHANGE;
+
 	return 1;
 
  too_short:
@@ -609,23 +612,6 @@
 	return 0;
 }
 
-/* This function is used to validate the arguments passed to a "payload" fetch
- * keyword. This keyword expects two positive integers, with the second one
- * being strictly positive. It is assumed that the types are already the correct
- * ones. Returns 0 on error, non-zero if OK. If <err_msg> is not NULL, it will be
- * filled with a pointer to an error message in case of error, that the caller
- * is responsible for freeing. The initial location must either be freeable or
- * NULL.
- */
-static int val_payload(struct arg *arg, char **err_msg)
-{
-	if (!arg[1].data.uint) {
-		memprintf(err_msg, "payload length must be > 0");
-		return 0;
-	}
-	return 1;
-}
-
 /* This function is used to validate the arguments passed to a "payload_lv" fetch
  * keyword. This keyword allows two positive integers and an optional signed one,
  * with the second one being strictly positive and the third one being greater than
@@ -660,7 +646,7 @@
  * instance IPv4/IPv6 must be declared IPv4.
  */
 static struct sample_fetch_kw_list smp_kws = {ILH, {
-	{ "payload",             smp_fetch_payload,        ARG2(2,UINT,UINT),      val_payload,    SMP_T_CBIN, SMP_USE_L6REQ|SMP_USE_L6RES },
+	{ "payload",             smp_fetch_payload,        ARG2(2,UINT,UINT),      NULL,           SMP_T_CBIN, SMP_USE_L6REQ|SMP_USE_L6RES },
 	{ "payload_lv",          smp_fetch_payload_lv,     ARG3(2,UINT,UINT,SINT), val_payload_lv, SMP_T_CBIN, SMP_USE_L6REQ|SMP_USE_L6RES },
 	{ "rdp_cookie",          smp_fetch_rdp_cookie,     ARG1(0,STR),            NULL,           SMP_T_CSTR, SMP_USE_L6REQ },
 	{ "rdp_cookie_cnt",      smp_fetch_rdp_cookie_cnt, ARG1(0,STR),            NULL,           SMP_T_UINT, SMP_USE_L6REQ },
@@ -671,14 +657,14 @@
 	{ "req_ssl_ver",         smp_fetch_req_ssl_ver,    0,                      NULL,           SMP_T_UINT, SMP_USE_L6REQ },
 
 	{ "req.len",             smp_fetch_req_len,        0,                      NULL,           SMP_T_UINT, SMP_USE_L6REQ },
-	{ "req.payload",         smp_fetch_payload,        ARG2(2,UINT,UINT),      val_payload,    SMP_T_CBIN, SMP_USE_L6REQ },
+	{ "req.payload",         smp_fetch_payload,        ARG2(2,UINT,UINT),      NULL,           SMP_T_CBIN, SMP_USE_L6REQ },
 	{ "req.payload_lv",      smp_fetch_payload_lv,     ARG3(2,UINT,UINT,SINT), val_payload_lv, SMP_T_CBIN, SMP_USE_L6REQ },
 	{ "req.rdp_cookie",      smp_fetch_rdp_cookie,     ARG1(0,STR),            NULL,           SMP_T_CSTR, SMP_USE_L6REQ },
 	{ "req.rdp_cookie_cnt",  smp_fetch_rdp_cookie_cnt, ARG1(0,STR),            NULL,           SMP_T_UINT, SMP_USE_L6REQ },
 	{ "req.ssl_hello_type",  smp_fetch_ssl_hello_type, 0,                      NULL,           SMP_T_UINT, SMP_USE_L6REQ },
 	{ "req.ssl_sni",         smp_fetch_ssl_hello_sni,  0,                      NULL,           SMP_T_CSTR, SMP_USE_L6REQ },
 	{ "req.ssl_ver",         smp_fetch_req_ssl_ver,    0,                      NULL,           SMP_T_UINT, SMP_USE_L6REQ },
-	{ "res.payload",         smp_fetch_payload,        ARG2(2,UINT,UINT),      val_payload,    SMP_T_CBIN, SMP_USE_L6RES },
+	{ "res.payload",         smp_fetch_payload,        ARG2(2,UINT,UINT),      NULL,           SMP_T_CBIN, SMP_USE_L6RES },
 	{ "res.payload_lv",      smp_fetch_payload_lv,     ARG3(2,UINT,UINT,SINT), val_payload_lv, SMP_T_CBIN, SMP_USE_L6RES },
 	{ "res.ssl_hello_type",  smp_fetch_ssl_hello_type, 0,                      NULL,           SMP_T_UINT, SMP_USE_L6RES },
 	{ "wait_end",            smp_fetch_wait_end,       0,                      NULL,           SMP_T_BOOL, SMP_USE_INTRN },