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

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: http://postgr.es/m/Yqbc8bAdwnP02na4@paquier.xyz