Allow forwarding of CLTR-C (SIGINT) to the remote process.

This feature is controlled by the env.remote_interupt variable. The default
value is None, which means that it depends on the value of the "invoke_shell"
parameter.

If no PTY is allocated, then remote interrupts will always be disabled.

If remote interrupts are enabled, then a CTRL-C pressed on the local terminal will deliver a SIGINT to the remote process, rather than terminating the local Fabric IO loop.