hash-func: add generic hash_ops implementation for hashing paths

This is similar to string_hash_ops but operates one file system paths
specifically. It will ensure that "/foo//bar" and "///foo/bar" are
considered to be the same path for hashmap purposes.

This makes use of the existing path_compare() API, and adds a matching
hashing function for it.

Note that relative and absolute paths will hash to different values,
however whether the path is suffixed with a slash or not is not
detected. This matches the existing path_compare() behaviour, and
follows the logic that on Linux there can't be two different objects at
path /foo/bar and /foo/bar/ either.

src/basic/hash-funcs.h

@@ -36,9 +36,12 @@ void string_hash_func(const void *p, struct siphash *state);
 int string_compare_func(const void *a, const void *b) _pure_;
 extern const struct hash_ops string_hash_ops;
 
-/* This will compare the passed pointers directly, and will not
- * dereference them. This is hence not useful for strings or
- * suchlike. */
+void path_hash_func(const void *p, struct siphash *state);
+int path_compare_func(const void *a, const void *b) _pure_;
+extern const struct hash_ops path_hash_ops;
+
+/* This will compare the passed pointers directly, and will not dereference them. This is hence not useful for strings
+ * or suchlike. */
 void trivial_hash_func(const void *p, struct siphash *state);
 int trivial_compare_func(const void *a, const void *b) _const_;
 extern const struct hash_ops trivial_hash_ops;