BUG/MINOR: smtpchk: SMTP Service check should gracefully close SMTP transaction

BUG/MINOR: smtpchk: SMTP Service check should gracefully close SMTP transaction

At present option smtpchk closes the TCP connection abruptly on completion of service checking,
even if successful. This can result in a very high volume of errors in backend SMTP server logs.
This patch ensures an SMTP QUIT is sent and a positive 2xx response is received from the SMTP
server prior to disconnection.

This patch depends on the following one:

 * MINOR: smtpchk: Update expect rule to fully match replies to EHLO commands

This patch should fix the issue #1812. It may be backported as far as 2.2
with the commit above On the 2.2, proxy_parse_smtpchk_opt() function is
located in src/check.c

[cf: I updated reg-tests script accordingly]