MEDIUM: ssl: rename the struct "cert_key_and_chain" to "ckch_data"

Rename the structure "cert_key_and_chain" to "ckch_data" in order to
avoid confusion with the store whcih often called "ckchs".

The "cert_key_and_chain *ckch" were renamed "ckch_data *data", so we now
have store->data instead of ckchs->ckch.

Marked medium because it changes the API.
diff --git a/src/ssl_crtlist.c b/src/ssl_crtlist.c
index b5979bd..c532c01 100644
--- a/src/ssl_crtlist.c
+++ b/src/ssl_crtlist.c
@@ -1288,7 +1288,7 @@
 		memprintf(&err, "certificate '%s' does not exist!", cert_path);
 		goto error;
 	}
-	if (store->ckch == NULL || store->ckch->cert == NULL) {
+	if (store->data == NULL || store->data->cert == NULL) {
 		memprintf(&err, "certificate '%s' is empty!", cert_path);
 		goto error;
 	}
@@ -1383,7 +1383,7 @@
 		memprintf(&err, "certificate '%s' does not exist!", cert_path);
 		goto error;
 	}
-	if (store->ckch == NULL || store->ckch->cert == NULL) {
+	if (store->data == NULL || store->data->cert == NULL) {
 		memprintf(&err, "certificate '%s' is empty!", cert_path);
 		goto error;
 	}