implement the systemd notification protocol manually, drop libsystemd

implement the systemd notification protocol manually, drop 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/