Add basic domain name format validation

[PeterDaveHello]

Avoid wasting time on checking invalid domain

[thomasmerz]

Do you mean to eliminate "Block Page detected"…

Running dig/nslookup over 10 AD(and tracker)-blocking DNS:
 - 1Hosts (76.76.2.38) ... NextDNS Block Page detected!

…and only display "Failed!"?

 - AdGuard (94.140.14.14) ... OK! (telemetry.v.dropbox.com. 162.125.21.2)
 - AhaDNS (5.2.75.75) ... Failed!
   ** server can't find telemetry.dropbox.com: REFUSED
 - clean.dnsforge.de (49.12.223.2) ... Failed!
   Address: 0.0.0.0
   Address: ::

I totally agree and really would appreciate this because it caused my some time what is meant with "Block Page detected" 😄

[PeterDaveHello]

@thomasmerz nope, I only mean to verify the input 😅

[thomasmerz]

Oh, what a pity. 😆