libuvmem: usermode port of vmem(9)

The quantum cache is disabled, there is no uma.

Intent is to use this for resource allocation in bhyve(8), for start.
Addition of -luvmem to bhyve linking was done to test changes to share/mk.

Reviewed by: bnovkov, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27220