BUG/MINOR: check: do not reset check flags on purge

Do not reset check flags when setting CHK_ST_PURGE.

Currently, this change has no impact. However, it is semantically wrong
to clear important flags such as CHK_ST_AGENT on purge.

Furthermore, this change will become mandatoy for a future fix to
properly free agent checks on dynamic servers removal. For this, it will
be needed to differentiate health/agent-check on purge via CHK_ST_AGENT
to properly free agent checks.

This must not be backported unless dynamic servers checks are
backported.
diff --git a/src/check.c b/src/check.c
index b1ffe5e..8ad32fe 100644
--- a/src/check.c
+++ b/src/check.c
@@ -1363,7 +1363,7 @@
  */
 void check_purge(struct check *check)
 {
-	check->state = CHK_ST_PURGE;
+	check->state |= CHK_ST_PURGE;
 	task_wakeup(check->task, TASK_WOKEN_OTHER);
 }