pgsql: Push enable/disable of notify and catchup interrupts all the way

Log Message:
-----------
Push enable/disable of notify and catchup interrupts all the way down
to just around the bare recv() call that gets a command from the client.
The former placement in PostgresMain was unsafe because the intermediate
processing layers (especially SSL) use facilities such as malloc that are
not necessarily re-entrant. Per report from counterstorm.com.

Modified Files:
--------------
pgsql/src/backend/libpq:
be-secure.c (r1.56 -> r1.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/be-secure.c.diff?r1=1.56&r2=1.57)
pgsql/src/backend/tcop:
postgres.c (r1.445 -> r1.446)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.445&r2=1.446)
pgsql/src/include/tcop:
tcopprot.h (r1.73 -> r1.74)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/tcop/tcopprot.h.diff?r1=1.73&r2=1.74)