OVSDriver: fix parent death race
OVSDriver: fix parent death race

Upcall processes use prctl(PR_SET_PDEATHSIG) to be notified when the main
process dies. There's a short window where the child process has started but
not yet run prctl. If the parent dies during this window the child won't
notice. I've seen this race occur when there's a crash during startup.

floodlight/ivs,floodlight/ivs,vezril/ivs,vezril/ivs,floodlight/ivs,vezril/ivs