Since PG 12, clientcert no longer supported only on/off, so remove 1/0
as possible values, and instead support only the text strings
'verify-ca' and 'verify-full'.
Remove support for 'no-verify' since that is possible by just not
specifying clientcert.
Also, throw an error if 'verify-ca' is used and 'cert' authentication is
used, since cert authentication requires verify-full.
Also improve the docs.
THIS IS A BACKWARD INCOMPATIBLE API CHANGE.
Reported-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20200716.093012.1627751694396009053.horikyota.ntt@gmail.com
Author: Kyotaro Horiguchi
Backpatch-through: master
253f1025da