src/channel.scm (free-channel!): Remove.

* src/channel-type.c (ptob_close): Free the underlying SSH
  channel.
* src/channel.scm (free-channel!): Remove.
* src/channel-func.c (guile_ssh_channel_free): Remove.
channel-type.c (_ssh_channel_to_scm): Fix ptob flags.

* src/channel-type.c (_ssh_channel_to_scm): Don't set
  `SCM_BUFLINE' bit.