Add macro to switch to another wire to debug it
Add kill and spawn to wire_io

spawn is a wrapper around fork and exec to run a process, return the pid
and be able to hook into the stdin/stdout/stderr feeds.
Fix deadlock in wire_channel

Due to incorrect order to list_add we never really added the waiter to
the list to be woken up.
Improve wire_channel

Do not wait for the list to empty, just wait for our message to be
finished. We may reach a deadlock otherwise.