ssh2: add options to control idle connection lifetime

added two new options:

admin.ssh.idle-timeout =
admin.ssh.idle-timeout.unit =

to tell ssh how lond to keep idle connection

Acked-by: Albert Rossi
Acked-by: Paul Millar
Acked-by: Dmitry Litvintsev
Target: master
Require-book: yes
Require-notes: yes