nixos/zsh: Fix tramp support

For a long time, TRAMP has not worked with ZSH NixOS servers. I
thought I fixed this in 0740f57e63af61694d14796286cb9204, but now
realize that was only half the problem. For TRAMP to start working
again ‘unsetopt zle’ was needed, otherwise the connection would hang.
In addition, I have a few more settings added that can apparenty
interfere with these settings.

nixos/modules/programs/zsh/zsh.nix

@@ -69,9 +69,7 @@ in
 
       promptInit = mkOption {
         default = ''
-          if [ "$TERM" != dumb ]; then
           autoload -U promptinit && promptinit && prompt walters && setopt prompt_sp
-          fi
         '';
         description = ''
           Shell script code used to initialise the zsh prompt.
@@ -213,6 +211,15 @@ in
 
         ${cfg.promptInit}
 
+        # Need to disable features to support TRAMP
+        if [ "$TERM" = dumb ]; then
+            unsetopt zle prompt_cr prompt_subst
+            unfunction precmd preexec
+            unset RPS1 RPROMPT
+            PS1='$ '
+            PROMPT='$ '
+        fi
+
         # Read system-wide modifications.
         if test -f /etc/zshrc.local; then
             . /etc/zshrc.local