Splitting TXT and SPF records longer than 255 chars into more strings

- DNS defines that a single string can be of max length 255 chars
- However, there can be more of then
- So, if the tota lstring length is longer, it's split into more strings
Fixed SPF record to ba handled the same way as TXT record

- strings are quoted
Fixed checking the TXT/SPF record value size

- Record size can be bigger than 255 characters but in that
  case it needs to be split into more smaller parts
  e.g., "a" "b" "c" instead of "a b c"
- The old implementation expected the value size to be max 255
  in total, which was wrong

Conflicts:
	src/include/dns-server/dialog-masterzone.rb
Deleted file added by backporting a patch