session.scm (make-session): Improve.

* src/session-type.c (guile_ssh_make_session): Change Scheme name
  to `%make-session'.
* src/session.scm (make-session): New procedure.
session-func.c (guile_ssh_connect): Improve.

* src/session-func.c (guile_ssh_connect): Rename to
  `guile_ssh_connect_x'.  Throw an exception on error.
* src/session-func.h: Add `guile_ssh_connect_x'.
session-func.c (guile_ssh_authenticate_server): Improve.

* src/session-func.c (guile_ssh_authenticate_server): Throw an
  exception on error.
* src/session-func.h: Add `guile_ssh_authenticate_server'.
Merge branch 'master' into ssh-server-api