MEDIUM: config: add a "ciphers" keyword to set SSL cipher suites

This is supported for both servers and listeners. The cipher suite
simply follows the "ciphers" keyword.
diff --git a/include/types/protocols.h b/include/types/protocols.h
index 5c13569..4b4ef16 100644
--- a/include/types/protocols.h
+++ b/include/types/protocols.h
@@ -134,6 +134,7 @@
 	char *ssl_cert;			/* ssl certificate */
 	struct {
 		SSL_CTX *ctx;
+		char *ciphers;		/* cipher suite to use if non-null */
 	} ssl_ctx;
 #endif
 	/* warning: this struct is huge, keep it at the bottom */