Regression tests for LOCK TABLE.

Robins Tharakan, reviewed by Szymon Guz, substantially revised by me.
Assert that syscache lookups don't happen outside transactions.

Andres Freund
Tab completion for \lo_import

Josh Kupershmidt
Check get_tle_by_resno() result before deref

When creating a sort to support a group by, we need to look up the
target entry in the target list by the resno using get_tle_by_resno().
This particular code-path didn't check the result prior to attempting
to dereference it, while all other callers did.  While I can't see a
way for this usage of get_tle_by_resno() to fail (you can't ask for
a column to be sorted on which isn't included in the group by), it's
probably best to check that we didn't end up with a NULL somehow
anyway than risk the segfault.

I'm willing to back-patch this if others feel it's necessary, but my
guess is new features are what might tickle this rather than anything
existing.

Missing check spotted by the Coverity scanner.