Robustify fork server launching & problem detection, part 2

[copybara-service]

Robustify fork server launching & problem detection, part 2

This change addresses the problem (2) from the bug:

2. Both the engine and the fork server attempt to open the comms pipes in the blocking mode. That requires that both sides successfully arrive at the point where they attempt opening. However, a misbehaving or miscompiled fuzz target may exit and thus terminate the fork server early, leaving the engine perpetually waiting at an open() call.

This and the previous change fix the symptom (3):

3. As a result of (1) and (2), execution problems aren't properly detected: a bad target may silently fail without opening the pipes and the engine blocks at the open() call forever, waiting for the absent fork server to open() its side of the pipes.