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.
On 12/09/2014 02:19 PM, andrew-elder wrote:
> msgbuf still needs to be free'd doesn't it?

Oops! Talk about leaks...
Pushed.

(side note: hopefully coverity would have seen this - the defect count is now quite low on the talkers/listeners/client side)