path-util: rework file_in_same_dir() on top of path_extract_directory()

Let's port one more over.

Note that this changes behaviour of file_in_same_dir() in some regards.
Specifically, a trailing slash of the input path will be treated
differently: previously we'd operate below that dir then, instead of the
parent. I think that makes little sense however, and I think the code
using this function doesn't expect that either.

Moroever, addresses some corner cases if the path is specified as "/" or
".", i.e. where e cannot extract a parent. These will now be treated as
error, which I think is much cleaner.

src/basic/path-util.h

@@ -170,7 +170,7 @@ static inline bool path_is_safe(const char *p) {
 }
 bool path_is_normalized(const char *p) _pure_;
 
-char *file_in_same_dir(const char *path, const char *filename);
+int file_in_same_dir(const char *path, const char *filename, char **ret);
 
 bool hidden_or_backup_file(const char *filename) _pure_;