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/check.c b/src/check.c
index aedbed1..9ac66a5 100644
--- a/src/check.c
+++ b/src/check.c
@@ -1388,13 +1388,14 @@
 			    int nbcheck, int srvpos)
 {
 	struct task *t;
-	unsigned long thread_mask = MAX_THREADS_MASK;
 
+	/* task for the check. Process-based checks exclusively run on thread 1. */
 	if (check->type == PR_O2_EXT_CHK)
-		thread_mask = 1;
+		t = task_new_on(1);
+	else
+		t = task_new_anywhere();
 
-	/* task for the check */
-	if ((t = task_new(thread_mask)) == NULL) {
+	if (!t) {
 		ha_alert("Starting [%s:%s] check: out of memory.\n",
 			 check->server->proxy->id, check->server->id);
 		return 0;