Add --copy-file-range option to pg_upgrade.

The copy_file_range() system call is available on at least Linux and
FreeBSD, and asks the kernel to use efficient ways to copy ranges of a
file.  Options available to the kernel include sharing block ranges
(similar to --clone mode), and pushing down block copies to the storage
layer.

For automated testing, see PG_TEST_PG_UPGRADE_MODE.  (Perhaps in a later
commit we could consider setting this mode for one of the CI targets.)

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CA%2BhUKGKe7Hb0-UNih8VD5UNZy5-ojxFb3Pr3xSBBL8qj2M2%3DdQ%40mail.gmail.com

doc/src/sgml/ref/pgupgrade.sgml

@@ -263,6 +263,19 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--copy-file-range</option></term>
+      <listitem>
+       <para>
+        Use the <function>copy_file_range</function> system call for efficient
+        copying.  On some file systems this gives results similar to
+        <option>--clone</option>, sharing physical disk blocks, while on others
+        it may still copy blocks, but do so via an optimized path.  At present,
+        it is supported on Linux and FreeBSD.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-?</option></term>
       <term><option>--help</option></term>