smtpd.php: avoiding SMTP command false positive search in body/multipart...

When an e-mail body has a lot of content (eg. long and strange body or several attachments parts), checking for SMTP commands using if(stripos($input, 'MYCOMMAND') !== false) could trigger a false positive when MYCOMMAND actually is in the body (encountered 3 times over 10 different .png attachments, see below). This pull request proposes to force checking that commands exactly start exactly at the beginning of line of $input. This helped our server not mixing up attachments and commands.