pgsql: On Windows, force a checkpoint just before dropping a database's

Log Message:
-----------
On Windows, force a checkpoint just before dropping a database's physical
files and directories.  This ensures that the bgwriter will close any open
file references it is holding for files therein, which is needed for the
rmdir() to succeed.  Andrew Dunstan and Tom Lane.

Modified Files:
--------------
    pgsql/src/backend/commands:
        dbcommands.c (r1.145 -> r1.146)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.145&r2=1.146)
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.9&r2=1.10)