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uma: add UMA_ZONE_CONTIG, and a default contig_alloc

Description

uma: add UMA_ZONE_CONTIG, and a default contig_alloc

For now, copy the mbuf allocator.

Reviewed by: jeff, markj (previous version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23237