Add zpcpu_get_cpu() that converts base pointer of UMA_ZPCPU_ZONE
to a pointer private to a given cpuid.

Sponsored by:	Nginx, Inc.
Make M_ZERO flag work correctly on UMA_ZONE_PCPU zones.

Sponsored by:	Nginx, Inc.
Style.
Create two public UMA_ZONE_PCPU zones: 64 bit sized and pointer sized.

Sponsored by:	Nginx, Inc.
Expose real size of UMA allocations via libmemstat(3).

Sponsored by:	Nginx, Inc.