#47 Disable half of sessions menu command

[Andrew Stevenson]

For reasons similar to those described in the man page under further notes ("When using cssh on a large number of systems...") I often find I need to disable a subset of my active sessions, perform some commands, then do the same commands on the complement of the set (the remaining hosts).

To improve my workflow I added 2 more menu commands. The first disables half of the current hosts and the other enables all the hosts. In combination with the pre-existing toggle command I can achieve what I need easily.

Not sure how widely useful these will be but I attach a patch in case anyone is interested.