A setting to follow XDG Base Directory standard

XDG Base Directory is a standard for locations for storing various
files. Nix has a few files which seem to fit in the standard, but
currently use a custom location directly in the user's ~, polluting
it:

- ~/.nix-profile
- ~/.nix-defexpr
- ~/.nix-channels

This commit adds a config option (use-xdg-base-directories) to follow
the XDG spec and instead use the following locations:

- $XDG_STATE_HOME/nix/profile
- $XDG_STATE_HOME/nix/defexpr
- $XDG_STATE_HOME/nix/channels

If $XDG_STATE_HOME is not set, it is assumed to be ~/.local/state.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>
Co-authored-by: Tim Fenney <kodekata@gmail.com>
Co-authored-by: pasqui23 <pasqui23@users.noreply.github.com>
Co-authored-by: Artturin <Artturin@artturin.com>
Co-authored-by: John Ericson <Ericson2314@Yahoo.com>

src/libstore/profiles.cc

@@ -282,7 +282,7 @@ std::string optimisticLockProfile(const Path & profile)
 
 Path profilesDir()
 {
-    auto profileRoot = getDataDir() + "/nix/profiles";
+    auto profileRoot = createNixStateDir() + "/profiles";
     createDirs(profileRoot);
     return profileRoot;
 }
@@ -290,7 +290,7 @@ Path profilesDir()
 
 Path getDefaultProfile()
 {
-    Path profileLink = getHome() + "/.nix-profile";
+    Path profileLink = settings.useXDGBaseDirectories ? createNixStateDir() + "/profile" : getHome() + "/.nix-profile";
     try {
         auto profile =
             getuid() == 0