Allow auto_explain.log_min_duration to go up to INT_MAX.

The previous limit of INT_MAX / 1000 seems to have been cargo-culted in
from somewhere else.  Or possibly the value was converted to microseconds
at some point; but in all supported releases, it's just compared to other
values, so there's no need for the restriction.  This change raises the
effective limit from ~35 minutes to ~24 days, which conceivably is useful
to somebody, and anyway it's more consistent with the range of the core
log_min_duration_statement GUC.

Per complaint from Kevin Bloch.  Back-patch to all supported releases.

Discussion: https://postgr.es/m/8ea82d7e-cb78-8e05-0629-73aa14d2a0ca@codingthat.com

contrib/auto_explain/auto_explain.c

@@ -78,7 +78,7 @@ _PG_init(void)
 							"Zero prints all plans. -1 turns this feature off.",
 							&auto_explain_log_min_duration,
 							-1,
-							-1, INT_MAX / 1000,
+							-1, INT_MAX,
 							PGC_SUSET,
 							GUC_UNIT_MS,
 							NULL,