Add progress reporting to pg_checksums

This adds a new option to pg_checksums called -P/--progress, showing
every second some information about the computation state of an
operation for --check and --enable (--disable only updates the control
file and is quick).  This requires a pre-scan of the data folder so as
the total size of checksummable items can be calculated, and then it
gets compared to the amount processed.

Similarly to what is done for pg_rewind and pg_basebackup, the
information printed in the progress report consists of the current
amount of data computed and the total amount of data to compute.  This
could be extended later on.

Author: Michael Banck, Bernd Helmle
Reviewed-by: Fabien Coelho, Michael Paquier
Discussion: https://postgr.es/m/1535719851.1286.17.camel@credativ.de

doc/src/sgml/ref/pg_checksums.sgml

@@ -135,6 +135,17 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>-P</option></term>
+      <term><option>--progress</option></term>
+      <listitem>
+       <para>
+        Enable progress reporting. Turning this on will deliver a progress
+        report while checking or enabling checksums.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
        <term><option>-V</option></term>
        <term><option>--version</option></term>