Use libproc instead of lsof on Darwin to detect live gcroots

[booxter]

Motivation

On Darwin, lsof was used to detect gcroots. This is bad:

• lsof is very slow;
• lsof can't look into process environment.

It may result in gcroots removed while e.g. nix shell or nixpkgs vm with 9p
store is running.

For the latter, see also:
NixOS/nixpkgs#440579

Context

This is not happening on Lix because they switched to libproc circa 2024.

This PR largely adopts Lix implementation. With libproc, we can now read
envvars and hold gcroots mentioned there.

The Lix code is taken largely verbatim but was adopted to this codebase where
necessary. In addition, two bugs were identified in their implementation. Those
bugs are fixed in separate commits, for easier review and reference of Lix
folks in case they ever want to backport them.

The bugs are:

• empty argvs ("") were not properly handled, which may result in some envvars
  not consulted for possible gcroots;
• if a thread exited during the scan, the whole pid may be skipped for gcroot
  search, which may result in removing active roots.

The PR re-enables a previously disabled test case on darwin.

lsof is left for other platforms. Also, - a drive-by change - /proc is no
longer crawled on non-Linux platforms since the specific /proc layout is not
portable. (The /proc walk did not fail, so it's only a minor optimization /
correctness fix.)

Fixes #3011
Fixes #13990

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[booxter]

Deployed it to MacOS 26 laptop (backported relevant patches from the series to latest from unstable). Then validated with the throw-away test script as follows: (it's produced by Codex)

#!/usr/bin/env bash

set -euo pipefail

testDir=$(mktemp -d)
holder=

cleanup() {
    if [[ -n "$holder" ]] && kill -0 "$holder" 2>/dev/null; then
        kill "$holder" 2>/dev/null || true
        wait "$holder" 2>/dev/null || true
    fi
    rm -f "$testDir/payload" "$testDir/hold"
    rmdir "$testDir" 2>/dev/null || true
}
trap cleanup EXIT

holderBash=$(command -v bash)
printf 'gc-runtime-test-%s-%s\n' "$$" "$RANDOM" > "$testDir/payload"
root=$(/run/current-system/sw/bin/nix store add-file "$testDir/payload")
mkfifo "$testDir/hold"

echo "active-nix=$(/run/current-system/sw/bin/nix eval --impure --raw --expr builtins.nixVersion)"
echo "holder-bash=$holderBash"
echo "disposable-path=$root"

# The empty argument exercises parsing of empty argv entries. The FIFO keeps
# this unentitled, Nix-built Bash process alive without starting a child that
# inherits GC_RUNTIME_ROOT.
env -i GC_RUNTIME_ROOT="$root" \
    "$holderBash" -c 'read -r value < "$1"' '' "$testDir/hold" &
holder=$!

sleep 1
kill -0 "$holder"

echo "delete-while-live:"
if /run/current-system/sw/bin/nix-store --delete "$root"; then
    echo "FAIL: Nix deleted a runtime-rooted path"
    exit 1
fi

test -e "$root"
echo "GOOD: deletion refused while holder was alive"

kill "$holder"
wait "$holder" 2>/dev/null || true
holder=

echo "delete-after-exit:"
/run/current-system/sw/bin/nix-store --delete "$root"
test ! -e "$root"

echo "PASS: Darwin environment runtime-root discovery works"

> /tmp/test-nix-darwin-runtime-root.sh
active-nix=2.35.2+5
holder-bash=/etc/profiles/per-user/ihrachyshka/bin/bash
disposable-path=/nix/store/xm749ai71r4v3dzcb67qwc1jxf72h4c2-payload
delete-while-live:
finding garbage collector roots...
deleting specified paths...
0 store paths deleted, 0.0 KiB freed
error: Cannot delete path '/nix/store/xm749ai71r4v3dzcb67qwc1jxf72h4c2-payload' since it is still alive. To find out why, use: nix-store --query --roots and nix-store --query --referrers
GOOD: deletion refused while holder was alive
delete-after-exit:
finding garbage collector roots...
deleting specified paths...
deleting '/nix/store/xm749ai71r4v3dzcb67qwc1jxf72h4c2-payload'
deleting unused links...

[booxter]

MacOS CI failure is in filetransfer-retry-backoff and looks unrelated.