Cygwin: simplify linkat with AT_EMPTY_PATH

Cygwin: simplify linkat with AT_EMPTY_PATH

linkat(olddirfd, oldpath, oldname, newdirfd, newname, AT_EMPTY_PATH)
is supposed to create a link to the file referenced by olddirfd if
oldname is the empty string.  Currently this is done via the /proc
filesystem by converting the call to

  linkat(AT_FDCWD, "/proc/self/fd/<olddirfd>", newdirfd, newname,
         AT_SYMLINK_FOLLOW),

which ultimately leads to a call to the appropriate fhandler's link
method.  Simplify this by using cygheap_fdget to obtain the fhandler
directly.