Rename XXX_SINGLE_CIPHERSUITE -> XXX_CONF_SINGLE_CIPHERSUITE

This is in line with the other configurations options used
to hardcoded aspects of the SSL configuration.
diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 3ceae8c..63a3a2d 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -621,10 +621,10 @@
     mbedtls_ssl_conf_rng( &conf, mbedtls_ctr_drbg_random, &ctr_drbg );
     mbedtls_ssl_conf_dbg( &conf, my_debug, stdout );
 
-#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE)
     if( opt.force_ciphersuite[0] != DFL_FORCE_CIPHER )
         mbedtls_ssl_conf_ciphersuites( &conf, opt.force_ciphersuite );
-#endif /* !MBEDTLS_SSL_SINGLE_CIPHERSUITE */
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE */
 
     mbedtls_ssl_conf_ca_chain( &conf, &cacert, NULL );
     if( ( ret = mbedtls_ssl_conf_own_cert( &conf, &clicert, &pkey ) ) != 0 )