Added a new environment variable NORNS_DEBUG_CONFIG_FILE_OVERRIDE that
allows test programs to override the load process for the default
configuration file. When setting this variable, the test program is
expected to call either libnorns_reload_config_file() or
libnornsctl_reload_config_file() to set the configuration file for the
test explicitly.

This commit closes #3.

This commit fixes closes #2.

This commit closes #1

The new nornctl_status function queries the urd daemon and returns
summarized stats about all iotasks currently in the system. More
specifically, the function returns a nornsctl_stat_t structure that
contains the following information:
    st_running_tasks: Number of tasks currently running
    st_pending_tasks: Number of tasks currently pending
    st_eta: Estimated Time of Arrival of the currently running tasks
            (in seconds). Note that, internally, the daemon estimates
            how long a task will take by keeping track of the average
            namespace-to-namespace bandwidth from previous requests.
            This means that this field may contain NaN if no tasks
            have yet completed, since the daemon will not be able to
            estimate the E.T.A.

- 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.