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

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/bin/psql:
        command.c (r1.171 -> r1.172)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/command.c.diff?r1=1.171&r2=1.172)
        common.c (r1.125 -> r1.126)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/common.c.diff?r1=1.125&r2=1.126)
        copy.c (r1.66 -> r1.67)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/copy.c.diff?r1=1.66&r2=1.67)
        describe.c (r1.143 -> r1.144)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/describe.c.diff?r1=1.143&r2=1.144)
        input.c (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/input.c.diff?r1=1.58&r2=1.59)
        large_obj.c (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/large_obj.c.diff?r1=1.45&r2=1.46)
        mainloop.c (r1.82 -> r1.83)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/mainloop.c.diff?r1=1.82&r2=1.83)
        prompt.c (r1.47 -> r1.48)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/prompt.c.diff?r1=1.47&r2=1.48)
        settings.h (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/settings.h.diff?r1=1.28&r2=1.29)
        startup.c (r1.135 -> r1.136)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/startup.c.diff?r1=1.135&r2=1.136)
        variables.c (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/variables.c.diff?r1=1.25&r2=1.26)
        variables.h (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/variables.h.diff?r1=1.18&r2=1.19)