Flat file cleanup phase 2: make it work for pg_group. The flat group

Flat file cleanup phase 2: make it work for pg_group.  The flat group
file now identifies group members by usesysid not name; this avoids
needing to depend on SearchSysCache which we can't use during startup.
(The old representation was entirely broken anyway, since we did not
regenerate the file following RENAME USER.)  It's only a 95% solution
because if the group membership list is big enough to be toasted out
of line, we cannot read it during startup.  I think this will do for
the moment, until we have time to implement the planned pg_role
replacement for pg_group.