ipc: Fix handling of persistent option changes in a subprocess.

This addresses the bug causing kdesrc-build to warn repeatedly about a
git remote having moved (because it was comparing against a persistent
option set the normal way).

Unfortunately the persistent option update feature in the IPC code has
probably never worked properly as far as I can tell, so this may also
fix other minor bugs.

What I'd like to do before merging this all is to find a way to
integrate into the test suite somehow.