pgsql: Code review for psql multiline history patch(es).

Log Message:
-----------
Code review for psql multiline history patch(es).  Fix memory leak,
failure to enter commands in history if canceled by control-C, other
infelicities.

Modified Files:
--------------
    pgsql/src/bin/psql:
        command.c (r1.169 -> r1.170)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/command.c.diff?r1=1.169&r2=1.170)
        help.c (r1.110 -> r1.111)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/help.c.diff?r1=1.110&r2=1.111)
        input.c (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/input.c.diff?r1=1.53&r2=1.54)
        input.h (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/input.h.diff?r1=1.27&r2=1.28)
        mainloop.c (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/mainloop.c.diff?r1=1.78&r2=1.79)
        prompt.c (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/prompt.c.diff?r1=1.44&r2=1.45)
        tab-complete.c (r1.152 -> r1.153)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c.diff?r1=1.152&r2=1.153)