BUG/MEDIUM: email-alert: don't set server check status from a email-alert task

This avoids possible 100% cpu usage deadlock on a EMAIL_ALERTS_LOCK and
avoids sending lots of emails when 'option log-health-checks' is used.
It is avoided to change the server state and possibly queue a new email
while processing the email alert by setting check->status to
HCHK_STATUS_UNKNOWN which will exit the set_server_check_status(..) early.

This needs to be backported to 1.8.
diff --git a/src/checks.c b/src/checks.c
index eaf84a2..3a6f020 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -3145,7 +3145,7 @@
 			t->expire             = now_ms;
 			check->server         = alert->srv;
 			check->tcpcheck_rules = &alert->tcpcheck_rules;
-			check->status         = HCHK_STATUS_INI;
+			check->status         = HCHK_STATUS_UNKNOWN; // the UNKNOWN status is used to exit set_server_check_status(.) early
 			check->state         |= CHK_ST_ENABLED;
 		}