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.

tools.c

@@ -998,6 +998,37 @@ int tool_has_wildcards(char *path)
   return(0);
 }
 
+/*
+ * tool_parent_path
+ *
+ * Purpose:
+ *   Copies the parent directory part of a DOS/NetWare path.  The result is
+ *   uppercased and keeps a trailing volume colon for paths like SYS:FILE.TXT.
+ */
+void tool_parent_path(char *dst, char *src, int max)
+{
+  char tmp[260];
+  char *p;
+
+  tool_upcopy(tmp, src, sizeof(tmp));
+  p = strrchr(tmp, '\\');
+  if (!p) p = strrchr(tmp, ':');
+
+  if (!p) {
+    dst[0] = '\0';
+    return;
+  }
+
+  if (*p == ':') {
+    p++;
+    *p = '\0';
+  } else {
+    *p = '\0';
+  }
+
+  strmaxcpy(dst, tmp, max - 1);
+}
+
 /*
  * tool_parent_pattern
  *