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,

--
Rushabh Lathia
www.EnterpriseDB.com