lib/cli: encodeGNUCommandLine -> toGNUCommandLineShell

The semantic difference between `encode` and `to` is not apparent.
Users are likely to confuse both functions (which leads to unexpected
error messages about the wrong types). Like in `generators.nix`, all
functions should be prefixed by `to`.

Furthermore, converting to a string depends on the target context. In
this case, it’s a POSIX shell, so we should name it that (compare
`escapeShellArg` in `strings.nix`).

We can later add versions that escape for embedding in e.g. python
scripts or similar.

lib/tests/misc.nix

@@ -441,9 +441,34 @@ runTests {
     expected  = "«foo»";
   };
 
-  testRenderOptions = {
+
+# CLI
+
+  testToGNUCommandLine = {
     expr =
-       encodeGNUCommandLine
+       cli.toGNUCommandLine
+         { }
+         { data = builtins.toJSON { id = 0; };
+
+           X = "PUT";
+
+           retry = 3;
+
+           retry-delay = null;
+
+           url = [ "https://example.com/foo" "https://example.com/bar" ];
+
+           silent = false;
+
+           verbose = true;
+         };
+
+    expected = [ "-X" "PUT" "--data" "{\"id\":0}" "--retry" "3" "--url" "https://example.com/foo" "--url" "https://example.com/bar" "--verbose" ];
+  };
+
+  testToGNUCommandLineShell = {
+    expr =
+       cli.toGNUCommandLineShell
          { }
          { data = builtins.toJSON { id = 0; };