mailpit-299 [fix] Adding dot stuffing for POP3

[henningp]

Fixes issue 299 by adding dot stuffing to the POP3 server.

From the POP3 spec:

When all lines of the response have been sent, a
final line is sent, consisting of a termination octet (decimal code
046, ".") and a CRLF pair. If any line of the multi-line response
begins with the termination octet, the line is "byte-stuffed" by
pre-pending the termination octet to that line of the response.

In practice, this means that any message line starting with a dot must have an extra dot added at the start of the line. (The same technique is used in SMTP.)

Note: This is my first line of Golang. It does work, but I am not sure about its style.