systemctl: add "edit --stdin"

This is a fancy wrapper around "cat <<EOF", but:
- the user doesn't need to figure out the file name,
- parent directories are created automatically,
- daemon-reload is implied,
so it's a convenient way to create units or drop-ins.

Closes https://github.com/systemd/systemd/issues/21862.

src/shared/edit-util.h

@@ -24,7 +24,8 @@ struct EditFileContext {
         const char *marker_start;
         const char *marker_end;
         bool remove_parent;
-        bool overwrite_with_origin; /* whether to always overwrite target with original file */
+        bool overwrite_with_origin; /* Always overwrite target with original file. */
+        bool stdin;                 /* Read contents from stdin instead of launching an editor. */
 };
 
 void edit_file_context_done(EditFileContext *context);