[9.20] rem: pkg: Implement the systemd notification protocol manually to remove depen...

[9.20] rem: pkg: Implement the systemd notification protocol manually to remove dependency on libsystemd.

libsystemd, despite being useful, adds a huge surface area for just
using the sd_notify API. libsystemd's surface has been exploited in the
past [1].

Implement the systemd notification protocol by hand since it is just
sending newline-delimited datagrams to a UNIX socket. The code shouldn't
need more attention in the future since the notification protocol is
covered under systemd's stability promise [2].

We don't need to support VSOCK-backed service notifications since they
are only intended for virtual machine inits.

[1]: https://www.openwall.com/lists/oss-security/2024/03/29/4
[2]: https://systemd.io/PORTABILITY_AND_STABILITY/

Backport of MR https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/10263

Merge branch 'aydin/standalone-notification-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10454