Linux startup script offers graceful restart of workers (via SIGHUP)

Neither celeryd nor the existing startup script currently offer a convenient way to send a SIGHUP to each of the worker processes, so this pull request adds the capability to the startup script.

Rather than refactor the startup script to use common methods for reading pid files, I took a narrow, additive approach, to avoid conflicts with other startup script work and preserve the existing echoed messages.