Detached mode

[LachlanGunn]

The daemon should be able to run in detached mode. Clearly this means that using stdout is no longer possible, though obviously that isn't really acceptable in production anyway.

• Provide an I/O mechanism (see Better notification system #4)
• Write code for the actual detachment.
• Add the facility for a pidfile.
• Add some kind of remote control ability—handling SIGHUP at least, but a dedicated control connection would be more cross-platform.
• Write documentation.