Bug #3214: "helperHandleRead: unexpected read from ssl_crtd" errors.

Bug #3214: "helperHandleRead: unexpected read from ssl_crtd" errors.

Squid would read the beginning of a crtd response split across multiple
read operations and treat it as a complete response, causing various
certificate-related errors.

This patch:
 - allow the use of other than the '\n' character as the end of message mark
   for helper responses.
 - Use the '\1' char as end-of-message char for crtd helper. This char looks
   safe because the crtd messages are clear text only messages.