pgsql: Disallow TRUNCATE when there are any pending after-trigger events

Log Message:
-----------
Disallow TRUNCATE when there are any pending after-trigger events for
the target relation(s).  There might be some cases where we could discard
the pending event instead, but for the moment a conservative approach
seems sufficient.  Per report from Markus Schiltknecht and subsequent
discussion.

Modified Files:
--------------
    pgsql/src/backend/commands:
        tablecmds.c (r1.201 -> r1.202)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.201&r2=1.202)
        trigger.c (r1.206 -> r1.207)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.206&r2=1.207)
    pgsql/src/include/commands:
        trigger.h (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h.diff?r1=1.58&r2=1.59)