pgsql: Avoid passing zero as a value for vacuum_cost_limit, because it's

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.153 -> r2.154)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml.diff?r1=2.153&r2=2.154)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.47 -> r1.48)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.47&r2=1.48)