jobs: Refactor exiting jobs

jobs: Refactor exiting jobs

The new termination function can allow the restart
because it doesn't abort the waiting jobs.

So after calling 'jobs_exit', all threads stop.
The function 'job_start' returns.

The threads that are in blocking state, i.e. in
a call to 'jobs_enter' or 'jobs_call' are stopped.
An error status -1 with errno=EINTR is returned in
that case.

But before returning, that function calls the exit
handler if any.

Change-Id: I85a4b1976b09b18804eb681af940531ae5ace6c3
Signed-off-by: José Bollo <jose.bollo@iot.bzh>