job.c: add job_lock/unlock while keeping job.h intact

job.c: add job_lock/unlock while keeping job.h intact

With "intact" we mean that all job.h functions implicitly
take the lock. Therefore API callers are unmodified.

This means that:
- many static functions that will be always called with job lock held
  become _locked, and call _locked functions
- all public functions take the lock internally if needed, and call _locked
  functions
- all public functions called internally by other functions in job.c will have a
  _locked counterpart (sometimes public), to avoid deadlocks (job lock already taken).
  These functions are not used for now.
- some public functions called only from exernal files (not job.c) do not
  have _locked() counterpart and take the lock inside. Others won't need
  the lock at all because use fields only set at initialization and
  never modified.

job_{lock/unlock} is independent from real_job_{lock/unlock}.

Note: at this stage, job_{lock/unlock} and job lock guard macros
are *nop*

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20220926093214.506243-6-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>