SECURITY: Fix DoS on overly long input from Postfix

Thanks to Mateusz Jończyk who reported this issue and gave valuable
feedback for its resolution.

PostSRSd would hang on an overly long GET request, because the
fread()/fwrite() logic in the subprocess would get confused by the
remaining input line in its buffer.

Theoretically, this error should never occur, as Postfix is supposed to
send valid email addresses only, which are shorter than the buffer, even
assuming every single character is percent-encoded. However, Postfix
sometimes does seem to send malformed request with multiple concatenated
email addresses. I'm not sure if there's a reliable way to trigger this
condition by an external attacker, but it is a security bug in PostSRSd
nevertheless.