FSF/Interleaf style clash detection

[It is probably abuse of xemacs-patches to send in tentative stuff like this but I am not sure and the feature deadline is close.] As I am writing this an XEmacs is building that passed compiling the attached filelock.c file without much trouble (--with-clash-detectio=yes). This file is a naive port of the FSF 20.2 version of filelock.c that implements the new link base file locking. It still needs 1. Testing 2. Looking over by somebody for which this is not the largest C contribution to XEmacs ever. 3. Looking over by the NT folk since they don't have symlynk 4. Changelogs. 5. Check that I didn't loose XEmacs improvements. (The old file used call1_in_buffer) Unfortunately my time is up (I need to be off to Rugby training in -20 minutes). I am hoping somebody can finish this before the deadline. Jan P.S. You need to add Fsystem_name to lisp.h