pgsql: Insert CHECK_FOR_INTERRUPTS calls into loops in dbsize.c, to

Log Message:
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Insert CHECK_FOR_INTERRUPTS calls into loops in dbsize.c, to ensure that
the various disk-size-reporting functions will respond to query cancel
reasonably promptly even in very large databases.  Per report from
Kevin Grittner.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        dbsize.c (r1.7.2.2 -> r1.7.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/dbsize.c?r1=1.7.2.2&r2=1.7.2.3)