Log Message:
Update ruby-unicorn to 5.1.0.

* http: TypedData C-API conversion
* various documentation updates
* doc: bump olddoc to ~> 1.2 for extra NNTP URL
* rack is optional at runtime, required for dev
* doc update for ClientShutdown exceptions class
* unicorn 5.1.0.pre1 - rack is optional, again
* doc: reference --keep-file-descriptors for "bundle exec"
* doc: further trimming to reduce noise
* use leftpad Linux syscall for speed!