system & jobs: Reverse link and acquiring events

- The new version of 'systemd_get_event_loop' dont depends
anymore on jobs but returns a unique systemd event loop
for any threads.

- The event loop of jobs now use system.h function instead
of the opposite.

- The function 'jobs_get_sd_event' is removed

- The function 'jobs_acquire_event_manager' is introduced.
It is designed to that the current thread can manipulate
the single event manager

Change-Id: I31fe48dfe0f2cfa4d468e49338d36fea6e7e8081
Signed-off-by: José Bollo <jose.bollo@iot.bzh>