kadm5: use rk_closesocket to close sockets

kadm5_c_destroy did not use rk_closesocket when cleaning up
the context.  This results in an exception on Windows since a
socket is not a file descriptor.

Change-Id: I9ebddad61f0199acb495a0773925df4f41e4fef2
(cherry picked from commit 6c4423cd46ae465393db97defcfabe187311b155)