Tidy/enhance certificate chain output code.

New function ssl_add_cert_chain which adds a certificate chain to
SSL internal BUF_MEM. Use this function in ssl3_output_cert_chain
and dtls1_output_cert_chain instead of partly duplicating code.

ssl/ssl.h

diff --git a/ssl/ssl.h b/ssl/ssl.h
index 4b7397f..b9325fe 100644
--- a/ssl/ssl.h
+++ b/ssl/ssl.h
@@ -2175,6 +2175,8 @@
 #define SSL_F_SSL3_SETUP_WRITE_BUFFER			 291
 #define SSL_F_SSL3_WRITE_BYTES				 158
 #define SSL_F_SSL3_WRITE_PENDING			 159
+#define SSL_F_SSL_ADD_CERT_CHAIN			 316
+#define SSL_F_SSL_ADD_CERT_TO_BUF			 317
 #define SSL_F_SSL_ADD_CLIENTHELLO_RENEGOTIATE_EXT	 298
 #define SSL_F_SSL_ADD_CLIENTHELLO_TLSEXT		 277
 #define SSL_F_SSL_ADD_CLIENTHELLO_USE_SRTP_EXT		 307