PSQL commands: \quit_if, \quit_unless

This patch adds two very simple psql commands: \quit_if and \quit_unless.

Each takes an optional string parameter and evaluates it for truthiness via
ParseVariableBool().

If a true-ish value is passed to \quit_if, psql will behave as if the user
had input \quit.

\quit_unless will do nothing if the value given was true-ish, and will
\quit in any other circumstances.

Examples below show the behavior:

$ psql postgres
psql (10devel)
Type "help" for help.

# \quit_if
# \quit_unless
$ psql postgres
psql (10devel)
Type "help" for help.

# \quit_if f
# \quit_if 0
# \quit_if false
# \quit_if 2
unrecognized value "2" for "\quit_if"; assuming "on"
$ psql postgres
psql (10devel)
Type "help" for help.

# \quit_unless 2
unrecognized value "2" for "\quit_unless"; assuming "on"
# \quit_unless f
$

The need for this patch has arisen from several scripts I've written
recently that were about 97% psql and 3% bash or similar glue language. In
those scripts, there's often a test that says "there is no work to do here,
and that is not an error". I could engineer an optional SELECT 1/0 to
generate an error, but then I have to make the bash script determine
whether that was an error-error or a no-everything-is-ok-error. I also
don't want to wrap hundreds of lines of SQL inside python docstrings (thus
losing syntax highlighting, etc) just to handle one if/then.

Many thanks to Pavel Stehule for brainstorming this one with me.