Fix Connection to Enterasys B2 Switch

Connecting to an Enterasys B2 Switch (using demo_simple.py) for an
interactive shell doesn't work:

    Connected. Getting Shell...
    DEBUG:paramiko.transport:[chan 1] Max packet in: 34816 bytes
    DEBUG:paramiko.transport:[chan 1] Max packet out: 16384
    bytes
    INFO:paramiko.transport:Secsh channel 1 opened.
    ERROR:paramiko.transport:Channel request for unknown
    channel 0
    *** Caught exception: <class 'paramiko.SSHException'>: Channel closed.

The channels gets opened with index 1 on client
(paramiko) side and index 0 on server (switch)
side.
Probably the switch doesn't support this and
replies to the pty request with a wrong channel
id. Because of that paramiko closes the
connection.

This can be solved (or worked around) easily by
initialising _channel_counter with 0.
This is what other clients, like openssh do.

I don't see a problem with initialising this
counter with 0.

Conflicts:
	paramiko/transport.py
Changelog re #266
Merge branch '1.13' into 1.14