Conditionally apply a timeout rather than a maximum count of interim responses

[notroj]

Rather than appling a maximum count, apply an overall timeout to
reading interim responses to match the read timeout, if a read timeout is configured and the NE_SESSFLAG_1XXTIMEOUT flag is set - which defaults to set (issue #94):

* src/ne_session.h (NE_SESSFLAG_1XXTIMEOUT): New flag.

* src/ne_session.c (ne_session_create): Set NE_SESSFLAG_1XXTIMEOUT to default on.

* src/ne_request.c (send_request): Use the session read timeout to avoid looping reading interim responses indefinitely, rather than hard-coding a maximum number of responses.

* test/request.c (fail_request_with_error): Set read timeout. (fail_excess_1xx): Adjust expected error, send more interim responses.