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/bits/unistd_ext.h

@@ -47,13 +47,4 @@ extern __pid_t gettid (void) __THROW;
 # define CLOSE_RANGE_CLOEXEC (1U << 2)
 #endif
 
-/* Close all file descriptors in the range FD up to MAX_FD.  The flag FLAGS
-   are define by the CLOSE_RANGE prefix.  This function behaves like close
-   on the range, but in a fail-safe where it will either fail and not close
-   any file descriptor or close all of them.  Gaps where the file descriptor
-   is invalid are ignored.   Returns 0 on successor or -1 for failure (and
-   sets errno accordingly).  */
-extern int close_range (unsigned int __fd, unsigned int __max_fd,
-			int __flags) __THROW;
-
 #endif /* __USE_GNU  */