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>

PVS V1028: cast result of operation instead of operands

If an overflow occurs before the cast, we can't fix it. If we cast
first, we can rely on integer promotion to make the result use the
appropriate size.

Change-Id: I7462e28422456c07f179f94d39c10c408d9bec36
Reviewed-on: https://review.haiku-os.org/c/1623
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>