pg_checksums: Use new routine to retrieve data of PG_VERSION

 pg_checksums: Use new routine to retrieve data of PG_VERSION
 
 Previously, attempting to use pg_checksums on a cluster with a control
 file whose version does not match with what thetool is able to support
 would lead to the following error:
 pg_checksums: error: pg_control CRC value is incorrect
 
 This is confusing, because it would look like the control file is
 corrupted.  However, the contents of the control file are correct,
 pg_checksums not being able to understand how the past control file is
 shaped.
 
 This commit adds a check based on PG_VERSION, using the facility added
 by cd0be131ba6f, using the same error message as some of the other
 frontend tools.  A note is added in the documentation about the major
 version requirement.
 
 Author: Michael Banck <mbanck@gmx.net>
 Discussion: https://postgr.es/m/68f1ff21.170a0220.2c9b5f.4df5@mx.google.com