Fix bug 151971 (kdesvn-build SVN aborts with IPC failure).

I was never able to pin down what was causing the bug, or to reliably reproduce it. So I went and completely ditched the old IPC code instead. :(

It has been rewritten using anonymous pipes and adding an extra process so that the update and build processes can both proceed unfettered.

Also added is a command line option (--no-async) to disable the IPC stuff. You can set it as a config file option as well (just called async), it defaults to enabled.

In addition the async mode is turned off unless performing both the update and build processes. In theory async mode should always work but I see no reason to test the issue unnecessarily.