tools: add shared parent path helper

Move the common parent path extraction helper into tools.c and declare it in
net.h.

Replace the local implementations in RIGHTS, NDIR and NWTESTS with
tool_parent_path(). The helper keeps the existing behavior of uppercasing the
path and preserving a trailing volume colon for parent paths such as SYS:.

No behavior change.

net.h

@@ -166,6 +166,7 @@ extern void  tool_header_path(char *out, char *path, int max);
 extern int   tool_is_dot_dir(char *name);
 extern void  tool_join_path(char *out, char *base, char *name, int max);
 extern int   tool_has_wildcards(char *path);
+extern void  tool_parent_path(char *dst, char *src, int max);
 extern void  tool_parent_pattern(char *dir, char *pattern, char *path,
                                  int maxdir, int maxpat);