sendmail: make it possible to pause after connection helper is started

sendmail: make it possible to pause after connection helper is started

If a non-starttls helper is in use, initial 220 response is processed by us,
not by helper.
Some servers consider us to be a spammer if we don't wait for it.

It is not in protocol, but it is a real-life problem.

The workaround in this patch is a magic envvar, $SMTP_ANTISPAM_DELAY:

...
	-H 'PROG ARGS'	Run connection helper. Examples:
		openssl s_client -quiet -tls1 -starttls smtp -connect smtp.gmail.com:25
		openssl s_client -quiet -tls1 -connect smtp.gmail.com:465
			$SMTP_ANTISPAM_DELAY: seconds to wait after helper connect
...

By using it, people can tweak sendmail behavior even if sendmail invocation
is buried in some scripts.

function                                             old     new   delta
packed_usage                                       30464   30497     +33
sendmail_main                                       1185    1206     +21
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 54/0)               Total: 54 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>