util-lib: introduce dirent-util.[ch] for directory entry calls

Also, move a couple of more path-related functions to path-util.c.

src/basic/path-util.h

@@ -105,3 +105,10 @@ char* dirname_malloc(const char *path);
 
 bool filename_is_valid(const char *p) _pure_;
 bool path_is_safe(const char *p) _pure_;
+
+char *file_in_same_dir(const char *path, const char *filename);
+
+bool hidden_file_allow_backup(const char *filename);
+bool hidden_file(const char *filename) _pure_;
+
+bool is_device_path(const char *path);