relative max-cache-size does not respect resource limits

Automatic cache sizing is currently based on amount of physical memory and does not take into account externally imposed limits, e.g. CGroups.

Reproducer:

systemd-run -p MemoryMax=1G -p MemorySwapMax=0 --user --same-dir -t named -g -c /dev/null

on my system prints out:

none:92: 'max-cache-size 90%' - setting to 57738MB (out of 64153MB)

It should use 1 GiB as configured above.

Respect external limits for amount of memory available to named.

Presumably min(uv_get_constrained_memory(), uv_get_total_memory()) should give us reasonable value. Available since libuv 1.29.

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