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

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.