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.cc

@@ -334,6 +334,7 @@ shared_info::initialize ()
       NtAllocateLocallyUniqueId (&luid);/* Initialize pid_src to a low    */
       InterlockedExchange (&pid_src,	/* random value to make start pid */
 		   luid.LowPart % 2048);/* less predictably               */
+      forkable_hardlink_support = 0;    /* 0: Unknown, 1: Yes, -1: No */
       /* Defer debug output printing the installation root and installation key
 	 up to this point.  Debug output except for system_printf requires
 	 the global shared memory to exist. */