fork will return negative numbers, including -1, on success

[timnewsham]

nextpid is a signed integer, and allocpid() will return negative values after 2^31 calls. It will return -1 after 2^32 calls. These pids are returned by fork(), where the distinguished value of -1 indicates a failure. At full bore on my machine it takes on the order of 5 years of continuous forks to do 2^31 forks. So after 10 years of running, rxv6 can erroneously indicate that fork has failed when it has not.