CLEANUP: connection: rename conn->mux_ctx to conn->ctx

We most often store the mux context there but it can also be something
else while setting up the connection. Better call it "ctx" and know
that it's the owner's context than misleadingly call it mux_ctx and
get caught doing suspicious tricks.
diff --git a/src/cli.c b/src/cli.c
index ae419b0..cc377fc 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -897,7 +897,7 @@
 		if (fdt.iocb == conn_fd_handler) {
 			conn_flags = ((struct connection *)fdt.owner)->flags;
 			mux = ((struct connection *)fdt.owner)->mux;
-			ctx = ((struct connection *)fdt.owner)->mux_ctx;
+			ctx = ((struct connection *)fdt.owner)->ctx;
 			li = objt_listener(((struct connection *)fdt.owner)->target);
 			sv = objt_server(((struct connection *)fdt.owner)->target);
 			px = objt_proxy(((struct connection *)fdt.owner)->target);
@@ -945,7 +945,7 @@
 				chunk_appendf(&trash, " fe=%s", li->bind_conf->frontend->id);
 
 			if (mux) {
-				chunk_appendf(&trash, " mux=%s mux_ctx=%p", mux->name, ctx);
+				chunk_appendf(&trash, " mux=%s ctx=%p", mux->name, ctx);
 				if (mux->show_fd)
 					mux->show_fd(&trash, fdt.owner);
 			}