Add an option for custom vchan buffer size, to override default 64k (for
each direction). This is especially useful when the other side of
connection is MirageOS based, because of limited memory and default
grant table size (128 entries).
1. If local process is started, report its exit code, instaed of remote
one. To get remote exit code, simply start qrexec-client-vm without
third argument (and connect its stdin/stdout with the other process some
other way).
2. Report process terminated by signal.
Don't pretend that process terminated by signal finished successfuly.
Copy shell behaviour of reporting it as 128+signum.
3. Do not wait() for any child process, just the one we expect. In case
of qrexec-client-vm the child process is started differently and
wait()ing on it inside main loop would break its exit code reporting.
FixesQubesOS/qubes-issues#2861
