Stub out `pa_start_child_for_read`.

As far as I can tell, the main suspect call is pa_start_child_for_read() in libpulsecore. As far as I can tell:

The only consumer appears to be the gconf module, which is hardcoded to execute gconf-helper.
Despite what the Tails bugs says, this does not appear to directly be exposed to any IPC calls.

Since gconf doesn't exist inside the sandbox this should be harmless, but I might as well stub it out anyway.