Support multiple occurances of the same argument. Use this for a new

flag -e to new-window, split-window, respawn-window, respawn-pane to
pass environment variables into the newly created process. From Steffen
Christgau in GitHub issue 1697.

spawn.c

@@ -293,6 +293,8 @@ spawn_pane(struct spawn_context *sc, char **cause)
 
 	/* Create an environment for this pane. */
 	child = environ_for_session(s, 0);
+	if (sc->environ != NULL)
+		environ_copy(sc->environ, child);
 	environ_set(child, "TMUX_PANE", "%%%u", new_wp->id);
 
 	/*