Improved IPv6 link local resolver error and add unit testing for parsing function

[phillip-stephens]

Resolves #512

Following the discussion on the issue #512, I wanted to improve the error messages around why a given line will fail to parse in /etc/resolv.conf. This PR:

• improves our parsing function to offer more granular errors
• Make parsing function operate on an io.Reader to enable easier unit-testing of the parsing
• Fix a bug found with the unit test where we couldn't parse an IPv6 address with a port, or certain shortened IPv6 addresses

Per man resolvctl, the dns section specifies that nameservers may take the form: "That is, the acceptable full formats are "111.222.333.444:9953%ifname#example.com" for IPv4 and "[1111:2222::3333]:9953%ifname#example.com" for IPv6."

We don't support domain names with nameservers in /etc/resolv.conf or per-interface nameservers.

Testing

Link-Local IPv6 Address

$ cat /etc/resolv.conf     
...
nameserver [2001:db8:3333:4444:5555:6666:7777:8888]:53%eth0

~/zdns on  phillip/512-improved-link-local-resolver-error ⌚ 10:42:36
$ ./zdns A prstephens.com
FATA[0000] ZDNS is unable to parse resolvers file. ZDNS only supports IPv4 and IPv6 addresses with an optional port,  either 111.222.333.444:9953 or [1111:2222::3333]:9953. Please either modify your /etc/resolv.conf file or use '--name-servers'. Error: could not parse IP address ([2001:db8:3333:4444:5555:6666:7777:8888]:53%eth0) from config. We do not support specifying link-local IPv6 addresses or per-interface nameservers 

Passing Domain Names with IP addresses

$ cat /etc/resolv.conf   
...
nameserver 1.1.1.1#cloudflare.com

~/zdns on  phillip/512-improved-link-local-resolver-error ⌚ 10:48:28
$ ./zdns A prstephens.com
FATA[0000] ZDNS is unable to parse resolvers file. ZDNS only supports IPv4 and IPv6 addresses with an optional port,  either 111.222.333.444:9953 or [1111:2222::3333]:9953. Please either modify your /etc/resolv.conf file or use '--name-servers'. Error: could not parse IP address (1.1.1.1#cloudflare.com) from config. We do not support specifying domain names for nameservers 

Base Case - Works

$ cat /etc/resolv.conf     
...
search stanford.edu
nameserver 1.1.1.1

~/zdns on  phillip/512-improved-link-local-resolver-error ⌚ 10:49:56
$ ./zdns A prstephens.com  
{"name":"prstephens.com","results":{"A":{"data":{"additionals":[{"flags":"","type":"EDNS0","udpsize":1232,"version":0}],"answers":[{"answer":"104.21.96.65","class":"IN","name":"prstephens.com","ttl":300,"type":"A"},{"answer":"172.67.173.251","class":"IN","name":"prstephens.com","ttl":300,"type":"A"}],"protocol":"udp","resolver":"1.1.1.1:53"},"duration":0.022647458,"status":"NOERROR","timestamp":"2025-03-12T10:49:59-07:00"}}}
00h:00m:00s; Scan Complete; 1 names scanned; 42.48 names/sec; 100.0% success rate; NOERROR: 1