Critical section

Critical section

New strategy for memory locking to decrease the number of call to
to un/lock memory when processing critical lvm functions.

Introducing functions for critical section.

Inside the critical section - memory is always locked.
When leaving the critical section, the memory stays locked
until memlock_unlock() is called - this happens with
sync_local_dev_names() and sync_dev_names() function call.

memlock_reset() is needed to reset locking numbers after fork
(polldaemon).

The patch itself is mostly rename:

memlock_inc  -> critical_section_inc
memlock_dec  -> critical_section_dec
memlock      -> critical_section

Daemons (clmvd, dmevent) are using memlock_daemon_inc&dec
(mlockall()) thus they will never release or relock memory they've
already locked memory.

Macros sync_local_dev_names() and sync_dev_names() are functions.
It's better for debugging - and also we do not need to add memlock.h
to locking.h header (for memlock_unlock() prototyp).