pgsql: Prevent integer overflows during units conversion when displaying

Log Message:
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Prevent integer overflows during units conversion when displaying a GUC
variable that has units. Per report from Stefan Kaltenbrunner.

Backport to 8.2. I also backported my patch of 2007-06-21 that prevented
comparable overflows on the input side, since that now seems to have enough
field track record to be back-patched safely. That patch included addition
of hints listing the available unit names, which I did not bother to strip
out of it --- this will make a little more work for the translators, but
they can copy the translation from 8.3, and anyway an untranslated hint
is better than no hint.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/utils/misc:
guc.c (r1.360.2.2 -> r1.360.2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.360.2.2&r2=1.360.2.3)