pgsql: Allow numeric_fac() to be interrupted, since it can take quite a

Log Message:
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Allow numeric_fac() to be interrupted, since it can take quite a while for
large inputs.  Also cause it to error out immediately if the result will
overflow, instead of grinding through a lot of calculation first.
Per gripe from Jim Nasby.

Tags:
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REL8_0_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        numeric.c (r1.81 -> r1.81.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/numeric.c.diff?r1=1.81&r2=1.81.4.1)