Core: slab allocator free pages defragmentation.

Large allocations from a slab pool result in free page blocks being fragmented,
eventually leading to a situation when no further allocation larger than a page
size are possible from the pool.  While this isn't a problem for nginx itself,
it is known to be bad for various 3rd party modules.  Fix is to merge adjacent
blocks of free pages in the ngx_slab_free_pages() function.

Prodded by Wandenberg Peixoto and Yichun Zhang.