psql - add SHOW_ALL_RESULTS option

Hello devs,

The attached patch implements a new SHOW_ALL_RESULTS option for psql,
which shows all results of a combined query (\;) instead of only the last
one.

This solves a frustration that intermediate results were hidden from view
for no good reason that I could think of.

For that, call PQsendQuery instead of (mostly not documented) PQexec, and
rework how results are processed afterwards.

Timing is moved to ProcessQueryResults to keep the last result handling
out of the measured time. I think it would not be a big deal to include
it, but this is the previous behavior.

In passing, refactor a little and add comments. Make function names about
results plural or singular consistently with the fact the it processes one
or several results. Change "PrintQueryResult" to "HandleQueryResult"
because it was not always printing something. Also add a HandleCopyResult
function, which makes the patch a little bigger by moving things around
but clarifies the code.

Code in "common.c" is actually a little shorter than the previous version.
From my point of view the code is clearer than before because there is
only one loop over results, not an implicit one within PQexec and another
one afterwards to handle copy.

Add a few tests for the new feature.

IMHO this new setting should be on by default: few people know about \; so
it would not change anything for most, and I do not see why those who use
it would not be interested by the results of all the queries they asked
for.

--
Fabien.