Improvements in psql hooks for variables

Hi,

Following the discussion on forbidding an AUTOCOMMIT off->on
switch mid-transaction [1], attached is a patch that let the hooks
return a boolean indicating whether a change is allowed.

Using the hooks, bogus assignments to built-in variables can
be dealt with more strictly.

For example, pre-patch behavior:

=# \set ECHO errors
=# \set ECHO on
unrecognized value "on" for "ECHO"; assuming "none"
=# \echo :ECHO
on

which has two problems:
- we have to assume a value, even though we can't know what the user meant.
- after assignment, the user-visible value of the variable diverges from its
internal counterpart (pset.echo in this case).

Post-patch:
=# \set ECHO errors
=# \set ECHO on
unrecognized value "on" for "ECHO"
\set: error while setting variable
=# \echo :ECHO
errors

Both the internal pset.* state and the user-visible value are kept unchanged
is the input value is incorrect.

Concerning AUTOCOMMIT, autocommit_hook() could return false to forbid
a switch when the conditions are not met.

Another user-visible effect of the patch is that, using a bogus value
for a built-in variable on the command-line becomes a fatal error
that prevents psql to continue.
This is not directly intended by the patch but is a consequence
of SetVariable() failing.

Example:
$ ./psql -vECHO=bogus
unrecognized value "bogus" for "ECHO"
psql: could not set variable "ECHO"
$ echo $?
1

The built-in vars concerned by the change are:

booleans: AUTOCOMMIT, ON_ERROR_STOP, QUIET, SINGLELINE, SINGLESTEP

non-booleans: ECHO, ECHO_HIDDEN, ON_ERROR_ROLLBACK, COMP_KEYWORD_CASE,
HISTCONTROL, VERBOSITY, SHOW_CONTEXT

We could go further to close the gap between pset.* and the built-in
variables,
by changing how they're initialized and forbidding deletion as Tom
suggests in [2], but if there's negative feedback on the above changes,
I think we should hear it first.

[1]
https://www.postgresql.org/message-id/f2cb5838-0ee9-4fe3-acc0-df77aeb7d4c7%40mm
[2]
https://www.postgresql.org/message-id/4695.1473961140%40sss.pgh.pa.us

Best regards,
--
Daniel Vérité
PostgreSQL-powered mailer: http://www.manitou-mail.org
Twitter: @DanielVerite