Error during restore - dump taken with pg_dumpall -c option

On master branch when we do pg_dumpall with -c option, I can see that
it also dumping the "DROP ROLE pg_signal_backend", which seems wrong.
Because when you restore the dump, its throwing an error
"ERROR:  cannot drop role pg_signal_backend because it is required by the database system".


dumpRoles()::pg_dumpall.c does have logic to not dump "CREATE ROLE"  if the
rolename starts with "pg_", but similar check is missing into dropRoles() function.

PFA patch, to fix the problem in the similar way its been handled into dumpRoles().

Thanks,


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