added OpenSSH version detection mechanism
function setup_master() should be driving "dsh -M"
"dsh -M" calls multiplex.setup_master()
added ControlPersist
allow setting "control_persist none"
print info about hanging process
no more multiplex keyword; manually start multiplexing

use dsh -M
persist may be passed on the dsh command-line
refer to section 3.4 about speeding up synctool
better place to refer to section 3.14
allow dsh -M only on master node
print if already running
Merge branch 'multiplex'