Avoid passing zero as a value for vacuum_cost_limit, because it's not a valid

Avoid passing zero as a value for vacuum_cost_limit, because it's not a valid
value for the vacuum code.  Instead, make zero signify getting the value from a
higher level configuration facility, just like -1 in the original coding.  We
still document that -1 is the value that disables the feature, to avoid
confusing the user unnecessarily.

Reported by Galy Lee in <200705310914.l4V9E6JA094603@wwwmaster.postgresql.org>;
per subsequent discussion.