MINOR: connection: replace conn_assign with conn_attach

MINOR: connection: replace conn_assign with conn_attach

We don't want to assign the control nor transport layers anymore
at the same time as the data layer, because it prevents one from
keeping existing settings when reattaching a connection to an
existing stream interface.

Let's have conn_attach() replace conn_assign() for this purpose.

Thus, conn_prepare() + conn_attach() do exactly the same as the
previous conn_assign().