smtp: fix processing of initial dot in data

RFC 5321 4.1.1.4 specifies the CRLF terminating the DATA command
should be taken into account when chasing the <CRLF>.<CRLF> end marker.
Thus a leading dot character in data is also subject to escaping.

Tests 911 and test server are adapted to this situation.
New tests 951 and 952 check proper handling of initial dot in data.

Closes #2304