MINOR: tcpcheck: Don't handle anymore in-progress send rules in tcpcheck_main

MINOR: tcpcheck: Don't handle anymore in-progress send rules in tcpcheck_main

The special handling of in-progress send rules at the begining of
tcpcheck_main() function can be removed. Instead, at the begining of the
tcpcheck_eval_send() function, we test is there is some data in the output
buffer. In this case, it means we are evaluating an unfinished send rule and
we can jump to the sending part, skipping the formatting part.

This patch is mandatory for a major fix on the checks and must be backported
as far as 2.2.