channel_signal_function for handling when request_exec process dies from signal

[enkore]

libssh exit_status is only set by the exit-status packet/request, which is only sent for regular process exits. If the remote process exits due to a signal, an exit-signal request is sent instead, which doesn't set exit-status (which then remains at -1).

I'm halfway between
"this kinda sounds like a libssh bug, ssh_channel_get_exit_status should return the standard unixy composite, like wait(2) does"
and
"I guess we have to add a default callback for this and add a separate Channel.get_exit_signal function."