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/shared_info.h

@@ -33,7 +33,7 @@ public:
 /* Data accessible to all tasks */
 
 
-#define CURR_SHARED_MAGIC 0x3a6025edU
+#define CURR_SHARED_MAGIC 0xc590e67eU
 
 #define USER_VERSION   1
 
@@ -51,7 +51,7 @@ class shared_info
   mtinfo mt;
   loadavginfo loadavg;
   LONG pid_src;
-  char prefer_forkable_hardlinks; /* single byte access always is atomic */
+  char forkable_hardlink_support; /* single byte access always is atomic */
 
   void initialize ();
   void init_obcaseinsensitive ();