contrib/dbmirror bug + doc fix.

Attached is a patch to contrib/dbmirror that fixes a bug that was causing
the postmaster to crash when the trigger was running on a table without a
primary key.

I've also updated the docs to explicitly say that tables need primary
keys.


Thanks

--
Steven Singer                                       ssinger@navtechinc.com
Aircraft Performance Systems                Phone:  519-747-1170 ext 282
Navtech Systems Support Inc.                AFTN:   CYYZXNSX SITA: YYZNSCR
Waterloo, Ontario                           ARINC:  YKFNSCR