resolved: include NSID support to DNS stub

resolved: include NSID support to DNS stub

This adds minimal support for RFC5001 NSID to the stub resolver. This
useful to identify systemd-resolved when talking to the stub resolver,
and distuingishing the packets resolved answers itself (where NSID is
now set) from those which it proxies 1:1 upstream (where NSID will not
be set, or set to whatever the upstream server has it set to).

The NSID chosen consist of two parts:

1. The first part is derived from /etc/machine-id and identifies the
   resolved instance in a stable way.

2. The second part is the fixed string ".resolved.systemd.io".

This thus maybe used for a veriety of checks:

a. Am I talking to a resolved stub?
b. Am I talking to the same stub as last time?
c. Am I talking to the local resolved?

Given that the first part leaks the identity of the system in away two
protections are in place:

I) The NSID is only included on the main stub, not the extra stub. The
   main stub has with a TTL of 1 and other protections a lot of safety
   in place that the datagrams never leave the local system, thus the
   identifying info is only accessible to the local system — but
   /etc/machine-id is accessible to local software anyway.

II) The NSID is hashed from /etc/machine-id in a non-invertable way, so
    that the machine ID itself isn't leaked, but only an identifier
    derived from it.

Example dig run:

```
$ dig +nsid localhost @127.0.0.53

; <<>> DiG 9.11.23-RedHat-9.11.23-1.fc33 <<>> +nsid localhost @127.0.0.53
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46917
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
; NSID: 35 33 64 34 61 34 66 63 32 31 32 65 34 31 61 30 39 66 30 39 65 33 32 34 63 64 64 38 30 36 32 33 2e 72 65 73 6f 6c 76 65 64 2e 73 79 73 74 65 6d 64 2e 69 6f ("53d4a4fc212e41a09f09e324cdd80623.resolved.systemd.io")
;; QUESTION SECTION:
;localhost.			IN	A

;; ANSWER SECTION:
localhost.		0	IN	A	127.0.0.1

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Do Nov 12 20:57:16 CET 2020
;; MSG SIZE  rcvd: 110
```