Add ssl_set_arc4_support()

Rationale: if people want to disable RC4 but otherwise keep the default suite
list, it was cumbersome. Also, since it uses a global array,
ssl_list_ciphersuite() is not a convenient place. So the SSL modules look like
the best place, even if it means temporarily adding one SSL setting.
diff --git a/tests/compat.sh b/tests/compat.sh
index 260b26f..b06c232 100755
--- a/tests/compat.sh
+++ b/tests/compat.sh
@@ -667,7 +667,7 @@
             exit 1;
     esac
 
-    P_SERVER_ARGS="server_port=$PORT server_addr=0.0.0.0 force_version=$MODE"
+    P_SERVER_ARGS="server_port=$PORT server_addr=0.0.0.0 force_version=$MODE arc4=1"
     O_SERVER_ARGS="-accept $PORT -www -cipher NULL,ALL -$MODE"
     G_SERVER_ARGS="-p $PORT --http"
     G_SERVER_PRIO="EXPORT:+NULL:+MD5:+PSK:+DHE-PSK:+ECDHE-PSK:+RSA-PSK:-VERS-TLS-ALL:$G_PRIO_MODE"