forkables: simplify disabling via shm

* Rename cygwin_shared->prefer_forkable_hardlinks to
  forkable_hardlink_support, with values
  0 for Unknown, 1 for Supported, -1 for Unsupported.
  Upon first dll loaded ever, dll_list::forkable_ntnamesize checks the
  /var/run/cygfork directory to both exist and reside on NTFS, setting
  cygwin_shared->forkable_hardlink_support accordingly.
* Replace enum forkables_needs by bool forkables_created: Set
  to True by request_forkables after creating forkable hardlinks.

winsup/cygwin/dll_init.h

@@ -87,17 +87,9 @@ struct dll
 class dll_list
 {
   /* forkables */
-  enum
-    {
-      forkables_unknown,
-      forkables_impossible,
-      forkables_disabled,
-      forkables_needless,
-      forkables_needed,
-      forkables_created,
-    }
-    forkables_needs;
+  bool forkables_supported ();
   DWORD forkables_dirx_size;
+  bool forkables_created;
   PWCHAR forkables_dirx_ntname;
   PWCHAR forkables_mutex_name;
   HANDLE forkables_mutex;
@@ -160,10 +152,11 @@ public:
   void cleanup_forkables ();
   bool setup_forkables (bool with_forkables)
   {
-    if (forkables_needs == forkables_impossible)
-      return true; /* short cut to not retry fork */
-    /* Once used, always use forkables in current process chain. */
-    if (forkables_needs != forkables_unknown)
+    if (!forkables_supported ())
+      return true; /* no need to retry fork */
+    if (forkables_created)
+      /* Once created, use forkables in current
+	 process chain on first fork try already. */
       with_forkables = true;
     if (with_forkables)
       request_forkables ();