add check for reattach-to-user-namespace

if reattach-to-user-name-space exists, run it, otherwise just run the shell

* create login-shell command to perform this check
* update tmux.conf to run login-shell instead of reattach-to-user-namespace directly

tmux/tmux.conf.symlink

@@ -1,4 +1,4 @@
-set -g default-command "reattach-to-user-namespace -l zsh"
+set -g default-command "login-shell"
 # tmux display things in 256 colors
 set -g default-terminal "tmux-256color-italic"
 set -g status-utf8 on