Make psql reject attempts to set special variables to invalid values.

Previously, if the user set a special variable such as ECHO to an
unrecognized value, psql would bleat but store the new value anyway, and
then fall back to a default setting for the behavior controlled by the
variable.  This was agreed to be a not particularly good idea.  With
this patch, invalid values result in an error message and no change in
state.

(But this applies only to variables that affect psql's behavior; purely
informational variables such as ENCODING can still be set to random
values.)

To do this, modify the API for psql's assign-hook functions so that they
can return an OK/not OK result, and give them the responsibility for
printing error messages when they reject a value.  Adjust the APIs for
ParseVariableBool and ParseVariableNum to support the new behavior
conveniently.

In passing, document the variable VERSION, which had somehow escaped that.
And improve the quite-inadequate commenting in psql/variables.c.

Daniel Vérité, reviewed by Rahila Syed, some further tweaking by me

Discussion: https://postgr.es/m/7356e741-fa59-4146-a8eb-cf95fd6b21fb@mm