pgsql: Add an optional parameter to pg_start_backup() that specifies

Log Message:
-----------
Add an optional parameter to pg_start_backup() that specifies whether to do
the checkpoint in immediate or lazy mode.  This is to address complaints
that pg_start_backup() takes a long time even when there's no need to minimize
its I/O consumption.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        backup.sgml (r2.123 -> r2.124)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.123&r2=2.124)
        func.sgml (r1.474 -> r1.475)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.474&r2=1.475)
    pgsql/src/backend/access/transam:
        xlog.c (r1.334 -> r1.335)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.334&r2=1.335)
    pgsql/src/backend/catalog:
        system_views.sql (r1.59 -> r1.60)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/system_views.sql?r1=1.59&r2=1.60)
    pgsql/src/include/catalog:
        catversion.h (r1.528 -> r1.529)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.528&r2=1.529)
        pg_proc.h (r1.540 -> r1.541)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.540&r2=1.541)