call setsid() in ProxyCommand subprocess

[ploxiln]

so that signals sent to the foreground process group
are not also received by the ProxyCommand
(it should be closed by the transport when the parent shuts down)

inspired by paramiko#1183

[ploxiln]

I think this will need to be avoided on windows where setsid() is not available.

[ploxiln]

I'm now thinking that maybe this should be optional anyway, maybe not all ProxyCommands should use setsid(). Also, since Python-3.2:

if start_new_session is true the setsid() system call will be made in the child process prior to the execution of the subprocess. (POSIX only)