vmem: fix race condition in chunk_alloc_default()

Protect "pool->arenas" with "pool->arenas_lock".
Move the call to chunk_recycle() outside chunk_alloc_core(),
because the lock is needed only when (pool->pool_id == 0).
Merge pull request #59 from ldorau/vmem-fix-race-condition-in-chunk_alloc_default

vmem: fix race condition in chunk_alloc_default()
doc: updated example for libpmem man page
common: fix typos
doc: reflect library reorganization

All man pages now match the intended library reorganization.
Since some of the examples are used to create the man pages,
they've been updated ahead of the rest of the tree and therefor
building all the examples is currently disabled to prevent build
failures.
obj: minor typo in example