box: fix custom delimiter for telnet connection

In order to give a user ability to use a delimiter symbol within a
code the real delimiter is user-provided 'delim' plus "\n".
Since telnet sends "\r\n" on line break, the updated expression
delim + "\n" could not be found in a sequence data+delim+"\r\n",
so delimiter feature did not work at all.
Added delim + "\r" check along with delim + "\n", that solves the
described problem and does not violate backward compatibility.

Closes #2027