Update chain building function.

Don't clear verification errors from the error queue unless
SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR is set.

If errors occur during verification and SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR
is set return 2 so applications can issue warnings.
(cherry picked from commit 2dd6976f6d02f98b30c376951ac38f780a86b3b5)

ssl/ssl.h

diff --git a/ssl/ssl.h b/ssl/ssl.h
index c6cd6a9..27dcb09 100644
--- a/ssl/ssl.h
+++ b/ssl/ssl.h
@@ -791,6 +791,8 @@
 #define SSL_BUILD_CHAIN_FLAG_CHECK		0x4
 /* Ignore verification errors */
 #define SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR	0x8
+/* Clear verification errors from queue */
+#define SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR	0x10
 
 /* Flags returned by SSL_check_chain */
 /* Certificate can be used with this session */