MINOR: task: provide 3 task_new_* wrappers to simplify the API

We'll need to improve the API to pass other arguments in the future, so
let's start to adapt better to the current use cases. task_new() is used:
  - 18 times as task_new(tid_bit)
  - 18 times as task_new(MAX_THREADS_MASK)
  - 2 times with a single bit (in a loop)
  - 1 in the debug code that uses a mask

This patch provides 3 new functions to achieve this:
  - task_new_here()     to create a task on the calling thread
  - task_new_anywhere() to create a task to be run anywhere
  - task_new_on()       to create a task to run on a specific thread

The change is trivial and will allow us to later concentrate the
required adaptations to these 3 functions only. It's still possible
to call task_new() if needed but a comment was added to encourage the
use of the new ones instead. The debug code was not changed and still
uses it.
diff --git a/src/hlua.c b/src/hlua.c
index df46349..baf503b 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -8251,9 +8251,9 @@
 	 * otherwise, inherit the current thread identifier
 	 */
 	if (state_id == 0)
-		task = task_new(MAX_THREADS_MASK);
+		task = task_new_anywhere();
 	else
-		task = task_new(tid_bit);
+		task = task_new_here();
 	if (!task)
 		goto alloc_error;
 
@@ -8941,7 +8941,7 @@
 	ctx->ctx.hlua_apptcp.flags = 0;
 
 	/* Create task used by signal to wakeup applets. */
-	task = task_new(tid_bit);
+	task = task_new_here();
 	if (!task) {
 		SEND_ERR(px, "Lua applet tcp '%s': out of memory.\n",
 		         ctx->rule->arg.hlua_rule->fcn->name);
@@ -9134,7 +9134,7 @@
 		ctx->ctx.hlua_apphttp.flags |= APPLET_HTTP11;
 
 	/* Create task used by signal to wakeup applets. */
-	task = task_new(tid_bit);
+	task = task_new_here();
 	if (!task) {
 		SEND_ERR(px, "Lua applet http '%s': out of memory.\n",
 		         ctx->rule->arg.hlua_rule->fcn->name);
@@ -9753,7 +9753,7 @@
 	 * We use the same wakeup function than the Lua applet_tcp and
 	 * applet_http. It is absolutely compatible.
 	 */
-	appctx->ctx.hlua_cli.task = task_new(tid_bit);
+	appctx->ctx.hlua_cli.task = task_new_here();
 	if (!appctx->ctx.hlua_cli.task) {
 		SEND_ERR(NULL, "Lua cli '%s': out of memory.\n", fcn->name);
 		goto error;