ssh/: Improve printing of Guile-SSH objects

* ssh/channel-type.c (print_channel): Print data type first.  Print
  `freed' state if the channel has been closed and freed.
* ssh/key-type.c (print_key): Print data type first.
* ssh/message-type.c (print_message): Improve.
* ssh/session-type.c (print_session): Improve a bit.
TODO: Update
README: Require GNU Guile 2.0

* README: Require GNU Guile 2.0.
* doc/guile-ssh.texi (Installation): Likewise.
* configure.ac: Remove checks related to GNU Guile 1.8.
* Makefile.am, ssh/Makefile.am: Likewise.
* doc/version.texi: Update.
* NEWS: Update.
Merge branch 'master' into gc-bug-fix
ssh/channel.scm (channel-get-session): New procedure

* ssh/channel-func.c (guile_ssh_channel_get_session): New procedure.
* ssh/channel-func.h (guile_ssh_channel_get_session): Likewise.
* ssh/channel.scm (channel-get-session): Export.
* tests/client-server.scm ("channel-get-session"): New TC.
* doc/api-channels.texi (Channels): Add description of
  `channel-get-session' procedure.
* NEWS: Update.