tree-wide: rename hidden_file to hidden_or_backup_file and optimize

In standard linux parlance, "hidden" usually means that the file name starts
with ".", and nothing else. Rename the function to convey what the function does
better to casual readers.

Stop exposing hidden_file_allow_backup which is rather ugly and rewrite
hidden_file to extract the suffix first. Note that hidden_file_allow_backup
excluded files with "~" at the end, which is quite confusing. Let's get
rid of it before it gets used in the wrong place.

src/basic/path-util.h

@@ -122,7 +122,6 @@ 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 hidden_or_backup_file(const char *filename) _pure_;
 
 bool is_device_path(const char *path);