smtp: Added support for the conversion of Unix newlines during mail send

Added support for the automatic conversion of Unix newlines to CRLF
during mail uploads.

Feature: http://curl.haxx.se/bug/view.cgi?id=1456
smtp: Fixed const'ness of nread parameter in Curl_smtp_escape_eob()

...and some comment typos!
docs: Updated for commit 4bd860a001 and SMTP Unix line ending conversion