misbehave on PPC64

There was a mix of ulong and uint casts/variables which caused
incorrect value to be passed to/retrieved from max_relay_log_size
and sql_slave_skip_counter.

This mix failed to work on big-endian PPC64 where sizeof(int)= 4,
sizeof(long)= 8. E.g. session_var(thd, uint)= 1 will in fact store
0x100000000.