bootctl: create loader.conf only if it doesn't exist yet

If the snippet aleady exists, don't do anything, as the file was already
installed then.

(This also reworks the code to create the file atomically)

Fixes: #5396

src/basic/fs-util.h

@@ -91,3 +91,9 @@ static inline void rmdir_and_free(char *p) {
         free(p);
 }
 DEFINE_TRIVIAL_CLEANUP_FUNC(char*, rmdir_and_free);
+
+static inline void unlink_and_free(char *p) {
+        (void) unlink(p);
+        free(p);
+}
+DEFINE_TRIVIAL_CLEANUP_FUNC(char*, unlink_and_free);