Give clear error message if file descriptors limit is reached

[mitchellh]

When the limit of file descriptors is reached, stud simply crashes, giving an error similar to error #38. Stunnel and others typically give an error that the file descriptor limit has been reached, and continue running (simply dropping that connection).

I believe Stud should take a similar route.

[jamwt]

Thanks for the bug report! I replicated the aggressive SIGABRT and committed 970b1bc to handle this without child death.