Fix Crash in dns module

[michaelortmann]

Found by: weweilep
Patch by: michaelortmann
Fixes: #901

One-line summary:
Prevent possible crash, seen under CentOS 8, don't overflow system resolver nsaddr_list; fix size of dns-servers; fix doc and default settings; enhance logging

Additional description (if needed):
Thanks vanosg for helping with this one
sizeof dns servers was 120 and weweilep noted in IRC: 3 ipv6 addresses with full ports could hit that, IE: [aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa]:65000 [aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa]:65000 [aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa]:65000

Test cases demonstrating functionality (if applicable):
BotA.conf:

loadmodule dns
set dns-servers "1.1.1.1 foo 9.9.9.9 8.8.8.8 4.2.2.1"

$ ./eggdrop -nt BotA.conf

Module loaded: dns             
WARNING: Invalid dns-server foo
WARNING: 5 dns-servers configured but MAXNS is 3
         Surplus dns-servers ignored

.status all
  Module: dns, v 1.2
    Async DNS resolver is active.
    DNS server list: 1.1.1.1:53 9.9.9.9:53 8.8.8.8:53
    Using 0 bytes of memory

[michaelortmann]

Note, that i can repeat the crash with eggdrop 1.8.4 and up to commit 35912be see #760 which was a change to dns.mod. Since this commit i can't repeat the crash. But this patch here doesn't look obsolete.

[vanosg]

@weweilep have you tried this yet? Any issues?

[weweilep]

I have now tested this.

Started with 4 nameservers, got a warning about limiting to 3. Uptime has properly reported without crash. .tcl dnslookup does not crash.