Resolve "Minor documentation issues"

Closes #143

See merge request dlb/dlb!31

- Add scripts usage
- Add faq about tracing
- Removed deprecated --dlb Mercurium flag

- Option type is now instrument_items_t
- Evens in tracing.h are now enum instead of macros to run more
    compiler checks

Fix #144

Implement specific cases for process mask getter and setter

Closes #145

See merge request dlb/dlb!30

Allow pid = 0 to refer to the current process

After some analysis, their performance is too similar and
spinlocks have a worse-case scenario fighting the OS scheduler

Barrier_detach no longer closes the shared memory. Now, attaching
or dettaching just modifies the number of participants.
Fixes race conditions with multiple finalizes, since the detach only
happens once.

Use case: dlb_run has no handler for shmem_barrier, child process
is killed and dlb_run must clean shmems

Fix #142

Close #123

If the callback pointer was not set, talp was being invoked in the
beginning and again inside add_mask.

The last_process and is_empty checks need to be done on the same
mutex region. shmem_finalize now receives the function pointer
to check if the shared memory is empty.

Sometimes processes are deregistered but we lose CPUs in the free mask.
We also want to reset some shmem statistics if possible.