It allows to turn off accumulation of small pool allocations into a big
preallocated chunk of memory.  This is useful for debugging memory access
with sanitizer, since such accumulation can cover buffer overruns from
being detected.

This structure cannot be allocated as a large block anyway, otherwise that will
result in infinite recursion, since each large allocation requires to allocate
another ngx_pool_large_t.

The room for the structure is guaranteed by the NGX_MIN_POOL_SIZE constant.

It deduplicates some code for allocations from memory pool.
No functional changes.

This fixes use-after-free memory access with enabled debug log
when pool->log is allocated as a large block.

The check became meaningless after refactoring in 2a92804f4109.
With the loop currently in place, "current" can't be NULL, hence
the check can be dropped.

Additionally, the local variable "current" was removed to
simplify code, and pool->current now used directly instead.

Found by Coverity (CID 714236).

Previously pool->current wasn't moved back to pool, resulting in blocks
not used for further allocations if pool->current was already moved at the
time of ngx_reset_pool().  Additionally, to preserve logic of moving
pool->current, the p->d.failed counters are now properly cleared.  While
here, pool->chain is also cleared.

This change is essentially a nop with current code, but generally improves
things.

Fixed nearby grammar errors.

*) now pools are aligned to 16 bytes
*) forbidden to set non-aligned pool sizes