Avoid using atooid for numerical comparisons which arent Oids

Avoid using atooid for numerical comparisons which arent Oids

The check for the number of roles in the target cluster for an upgrade
selects the existing roles and performs a COUNT(*) over the result.  A
value of one is the expected query result value indicating that only
the install user is present in the new cluster. The result was converted
with the function for converting a string containing an Oid into a numeric,
which avoids potential overflow but makes the code less readable since
it's not actually an Oid at all.

Discussion: https://postgr.es/m/41AB5F1F-4389-4B25-9668-5C430375836C@yesql.se