Add support for starting per-user spooler via D-Bus

[michaelrsweet]

For CUPS 3.0 there will be a per-user spooler that can be started via D-Bus. Rather than tunneling IPP over D-Bus, we will have a "StartServer" method that returns the Unix domain socket to use (default CUPS_SERVER other than the old system-wide "/var/run/cupsd") for subsequent IPP requests.

Tracking the cups-locald portion of this work in the cups-local project.

[michaelrsweet]

[master 1bd5fc03f] Add D-Bus support for finding user session cups-locald domain socket (Issue #69)