Add pg_amcheck, a CLI for contrib/amcheck.

This makes it a lot easier to run the corruption checks that are
implemented by contrib/amcheck against lots of relations and get
the result in an easily understandable format. It has a wide variety
of options for choosing which relations to check and which checks
to perform, and it can run checks in parallel if you want.

Mark Dilger, reviewed by Peter Geoghegan and by me.

Discussion: http://postgr.es/m/12ED3DA8-25F0-4B68-937D-D907CFBF08E7@enterprisedb.com
Discussion: http://postgr.es/m/BA592F2D-F928-46FF-9516-2B827F067F57@enterprisedb.com

Author: robertmhaas

doc/src/sgml/ref/allfiles.sgml

@@ -196,6 +196,7 @@ Complete list of usable sgml source files in this directory.
 <!ENTITY dropuser           SYSTEM "dropuser.sgml">
 <!ENTITY ecpgRef            SYSTEM "ecpg-ref.sgml">
 <!ENTITY initdb             SYSTEM "initdb.sgml">
+<!ENTITY pgamcheck          SYSTEM "pg_amcheck.sgml">
 <!ENTITY pgarchivecleanup   SYSTEM "pgarchivecleanup.sgml">
 <!ENTITY pgBasebackup       SYSTEM "pg_basebackup.sgml">
 <!ENTITY pgbench            SYSTEM "pgbench.sgml">