Synchronize the netmgr changes with main branch

Merged Ondřej Surý requested to merge ondrej/cherry-picked-netmgr-v9_16 into v9_16

This is cherry-picked branch with all changes that has been done to the main branch (sans the changes to bin/rndc/rndc, lib/isccc/, bin/named/controlconf.c, and lib/isc/httpd.c).

The lib/isc/netmgr is almost identical now (the extra stuff in main is non-related to netmgr, but to a coding style).

I believe this also closes #2042 (closed) and solves part of #2071 (closed).

Closes #2042 (closed)

Edited by Ondřej Surý

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