nixos/shells: support defining aliases beginning with a dash or plus

Both Zsh and Bash support aliases that begin with characters also used to
indicate options to the “alias” built-in command, as long as the alias
definition is preceeded by a double dash.

This allows, e.g, for “alias -- +x=chmod +x”.

nixos/modules/programs/bash/bash.nix

@@ -12,7 +12,7 @@ let
   cfg = config.programs.bash;
 
   bashAliases = concatStringsSep "\n" (
-    mapAttrsFlatten (k: v: "alias ${k}=${escapeShellArg v}")
+    mapAttrsFlatten (k: v: "alias -- ${k}=${escapeShellArg v}")
       (filterAttrs (k: v: v != null) cfg.shellAliases)
   );
 

nixos/modules/programs/zsh/zsh.nix

@@ -12,7 +12,7 @@ let
   opt = options.programs.zsh;
 
   zshAliases = concatStringsSep "\n" (
-    mapAttrsFlatten (k: v: "alias ${k}=${escapeShellArg v}")
+    mapAttrsFlatten (k: v: "alias -- ${k}=${escapeShellArg v}")
       (filterAttrs (k: v: v != null) cfg.shellAliases)
   );