Stdouts of instances is buffered

[robryk]

Instances' stdout is not a terminal, so C stdio library uses a fixed size buffer as opposed to single-line buffer. This causes some output to be lost when an instance segfault or terminates unexpectedly in some other fashion.

It seems that the simplest way to fix this is changing stdout's buffering in zeus_local's initialization (with setvbuf).

[robryk]

A workaround is to use stderr instead of stdout: stderr is always either unbuffered or line buffered, unless that is manually changed with setvbuf.