As reported in #4, when the root listener is closed, calling Accept
on mux'd listeners would block forever, instead of returning an error.
This is because of a bug introduced in b90740d.
This commit fixes#4 by selecting on both donec and connc of muxed
listeners.
Added a test case to guard against this issue.
If a matcher go-routine mateches a connection after the root
listener was closed, cmux will panic since all connection channels
will be closed. This commit avoids closing connection channels but
signals the matcher go-routines that the listener is closed via
a done channel.
Tested: Added a test case for this issue.
