ssh/session.scm (session-get): New procedure

* ssh/session-func.c (guile_ssh_session_get): New procedure.
* ssh/session-func.h (guile_ssh_session_get): Likewise.
* ssh/session.scm (session-get): New procedure.
* tests/session.scm ("session-get"): New TC.
* doc/api-sessions.texi (Sessions): Add description of `session-get'
  procedure.
* NEWS: Update.