Invent an assign-hook mechanism for psql variables similar to the one

Invent an assign-hook mechanism for psql variables similar to the one
existing for backend GUC variables, and use this to eliminate repeated
fetching/parsing of psql variables in psql's inner loops.  In a trivial
test with lots of 'select 1;' commands, psql's CPU time went down almost
10%, although of course the effect on total elapsed time was much less.
Per discussion about how to ensure the upcoming FETCH_COUNT patch doesn't
cost any performance when not being used.