bump backlog up to 5
Merge #443 r=@jiahuang: bump backlog up to 5

fixes https://github.com/tessel/runtime/issues/441

Apparently [this](http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/configtuning-kernel-limits.html) says that it should be 128 but that seems high and unlikely to be able to be supported by actual hardware. 

Thoughts @natevw?