Enable "set half inactive" and "set all active"

Adds "set half inactive" and "set all active" menu commands.

When using a large number of systems sometimes you need to avoid running
a command on all simultaneously (e.g. to avoid resource limits or
connection throttling). These commands allow you to enter the command on
all systems, disable half, hit return to actually execute the command,
then use the pre-existing toggle active state option and hit return for
the other half. To finish you can use "set all active" to get back to
where you started.

Copied from http://sourceforge.net/p/clusterssh/support-requests/47/