Improve daemon and library configuration

- Changed the format of the global service configuration file to a
YAML-like format
- Urd daemon is now able to read its configuration from the
newly-formatted configuration file.
- Companion libraries linorns and libnornsctl are now capable of
reading their configuration from the global service configuration file.
- Libraries now encapsulate their state in a context structure shared
by all threads rather than in globals.
- Tests are only built now if the --enable-tests option is passed
when invoking configure
- Tests updated to patch the libraries on the fly in order to use
case-by-case configuration files.
- Tests now only cleanup their internal data if the test succeeds.