core: Make the plugin directory configurable.
sysdbd.conf: Added 'PluginDir' config option.
sysdbd: Improved startup logging.

Don't log success until all initialization is actually done. Also, don't
enable the connection logger until before entering the frontend main loop.
frontend: Make UNIX sockets the default listener implementation.
sysdbd: Return with status=1 in case of an initialization or config error.
t/integration: Added simple framework for integration tests.

Added a test using very simple configuration snippets.
t/integration/mock_plugin: Added plugin for integration tests.
t/integration/simple_query: Added test doing a simple query.
t/Makefile: Added integration test script to EXTRA_DIST.
t/coverage.sh: Don't use --disable-shared.

Else, mock_plugin.so won't be available during 'make test'.