mrpdclient: mrpdclient_close() send a "BYE" before closing connection

The "BYE" message allows the mrpd daemon to free a client connection.
This commit thus prevent a leak risk by both sending this message and
closing the socket on client side.
Oh, sure. I'll do a push about that.
It's a common idiom (maybe in the Linux world?) for these kind of "I do check the ret code, it failed, but what can I do about it, let's try to go on, and report in the end that something went wrong in the process".
Here is an example from the Linux kernel networking code:
http://lxr.free-electrons.com/source/net/core/drop_monitor.c#L240
Another here:
http://lxr.free-electrons.com/source/arch/x86/kernel/signal.c#L381
etc.
Not perfect, sure, but still better than not checking the ret at all.