Report fork failures in fork_stress
Retry fork/clone syscalls that fail with EAGAIN.

Lately I've seen intermittent test failures related to fork_stress. It's easy to
reproduce similar failures by running, e.g., 4 parallel runs of 10 sequential
runs of fork_stress. This patch fixes those failures although at least one more
patch is needed to make other fork()s robust too.

It's unclear why these failures only emerged recently, but it could be since
we started running both 64-bit and 32-bit fork_stress tests. Since they run
with and without syscallbuf, there can be up to 4 copies of fork_stress running
at once (before, there could only be 2). (Each fork_stress creates 250 child
processes and then reaps them.)
Handle EAGAIN when spawning the initial tracee.