io: Refactor close_range and closefrom

Now that Hurd implementis both close_range and closefrom (f2c996597d),
we can make close_range() a base ABI, and make the default closefrom()
implementation on top of close_range().

The generic closefrom() implementation based on __getdtablesize() is
moved to generic close_range().  On Linux it will be overriden by
the auto-generation syscall while on Hurd it will be a system specific
implementation.

The closefrom() now calls close_range() and __closefrom_fallback().
Since on Hurd close_range() does not fail, __closefrom_fallback() is an
empty static inline function set by__ASSUME_CLOSE_RANGE.

The __ASSUME_CLOSE_RANGE also allows optimize Linux
__closefrom_fallback() implementation when --enable-kernel=5.9 or
higher is used.

Finally the Linux specific tst-close_range.c is moved to io and
enabled as default.  The Linuxism and CLOSE_RANGE_UNSHARE are
guarded so it can be built for Hurd (I have not actually test it).

Checked on x86_64-linux-gnu, i686-linux-gnu, and with a i686-gnu
build.

io/closefrom.c

@@ -16,19 +16,21 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <stdbool.h>
 #include <stdio.h>
+#include <sys/param.h>
 #include <unistd.h>
-#include <not-cancel.h>
 
 void
 __closefrom (int lowfd)
 {
-  int maxfd = __getdtablesize ();
-  if (maxfd == -1)
-    __fortify_fail ("closefrom failed to get the file descriptor table size");
+  int l = MAX (0, lowfd);
 
-  for (int i = 0; i < maxfd; i++)
-    if (i >= lowfd)
-      __close_nocancel_nostatus (i);
+  int r = __close_range (l, ~0U, 0);
+  if (r == 0)
+    return ;
+
+  if (!__closefrom_fallback (l, true))
+    __fortify_fail ("closefrom failed to close a file descriptor");
 }
 weak_alias (__closefrom, closefrom)