Fixed comm.cc:377: "fd_table[fd].halfClosedReader != NULL" assertion

Fixed comm.cc:377: "fd_table[fd].halfClosedReader != NULL" assertion

Client side must stop reading when switching to a tunnel mode. The old code
called low-level commSetSelect to stop reading, but that left Comm tables in
an inconsistent state, with the client side reader callback still scheduled.
Squid would assert when the tunnel called comm_read with its own callback.

The bug is unrelated to half-closed connections despite halfClosedReader
mentioned in the assertion text. The assertion means "no more than one active
reader per FD".