Improve version reporting in pgbench.
Commit
547f04e73 caused pgbench to start printing its version number,
which seems like a fine idea, but it needs a bit more work:
* Print the server version number too, when different.
* Print the PG_VERSION string, not some reconstructed approximation.
This patch copies psql's well-tested code for the same purpose.
Discussion: https://postgr.es/m/
1226654.
1624036821@sss.pgh.pa.us