Replaced CHANNEL_RESERVED and CHANNEL_IN_USE with three values,
CHANNEL_ALLOC_SENT_OPEN, CHANNEL_ALLOC_RECEIVED_OPEN, and
CHANNEL_ALLOC_ACTIVE.
(class ssh_connection): Deleted open_fallback. Renamed array
in_use to alloc_state.

Rev: src/connection.h:1.76.2.7