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.

sysdeps/unix/sysv/linux/closefrom_fallback.c

@@ -21,6 +21,8 @@
 #include <not-cancel.h>
 #include <stdbool.h>
 
+#if !__ASSUME_CLOSE_RANGE
+
 /* Fallback code: iterates over /proc/self/fd, closing each file descriptor
    that fall on the criteria.  If DIRFD_FALLBACK is set, a failure on
    /proc/self/fd open will trigger a fallback that tries to close a file
@@ -97,3 +99,5 @@ err:
   __close_nocancel (dirfd);
   return ret;
 }
+
+#endif