Tweak behavior of psql --single-transaction depending on ON_ERROR_STOP

This commit, in completion of 157f873, forces a ROLLBACK for
--single-transaction only when ON_ERROR_STOP is used when one of the
steps defined by -f/-c fails.  Hence, COMMIT is always used when
ON_ERROR_STOP is not set, ignoring the status code of the last action
taken in the set of switches specified by -c/-f (previously ROLLBACK
would have been issued even without ON_ERROR_STOP if the last step
failed, while COMMIT was issued if a step in-between failed as long as
the last step succeeded, leading to more inconsistency).

While on it, this adds much more test coverage in this area when not
using ON_ERROR_STOP with multiple switch patterns involving -c and -f
for query files, single queries and slash commands.

The behavior of ON_ERROR_STOP is arguably a bug, but there was no much
support for a backpatch to force a ROLLBACK on a step failure, so this
change is done only on HEAD for now.

Per discussion with Tom Lane and Kyotaro Horiguchi.

Discussion: https://postgr.es/m/Yqbc8bAdwnP02na4@paquier.xyz

doc/src/sgml/ref/psql-ref.sgml

@@ -584,7 +584,8 @@ EOF
         <application>psql</application> to issue a <command>BEGIN</command> command
         before the first such option and a <command>COMMIT</command> command after
         the last one, thereby wrapping all the commands into a single
-        transaction. If any of the commands fails, a
+        transaction. If any of the commands fails and the variable
+        <varname>ON_ERROR_STOP</varname> was set, a
         <command>ROLLBACK</command> command is sent instead. This ensures that
         either all the commands complete successfully, or no changes are
         applied.