libnetwork: Synchronize netresolv with NetBSD 9.3.

Some of the Haiku-specific modifications that were previously unmarked
are now clearly marked with #ifdef __HAIKU__.

BIND9-specific files containing private APIs never exposed in public
headers have been removed.

Tested with wget, curl, pkgman (Network Kit), WebPositive (WebKit/curl),
and Falkon (QtWebEngine/Chromium). All DNS-related operations seem
to still work just fine.

(One patch also imported from NetBSD trunk for GCC 12+ compatibility.)

Change-Id: I4a349577b24b4df008fd9cba5d3a322cd24397f1
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6879
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

libnetwork: Move from src/kits to src/system.

Properly speaking, this is part of POSIX and not of the Be-style
"kits", and so it should live in system/ alongside libroot.
No functional change intended.

Change-Id: I0fcf78a09c76e220ad4f1719d147978ef4a3bc52
Reviewed-on: https://review.haiku-os.org/726
Reviewed-by: Axel Dörfler <axeld@pinc-software.de>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>