The norns library and service now supports a new type of i/o task called
NORNS_IOTASK_REMOVE that enqueues the deletion of a resource from
a backend. As of right now, only the deletion of NORNS_LOCAL_PATHs is
supported.

This commit defines the NORNS_IOTASK_REMOVE subtype and also implements
the basic mechanism so that a task of this type can be executed.

First implementation of nornsctl_send_command()

The nornsctl_send_command() function allows administrative applications
such as SLURM to send control commands to the urd server to control its
behavior or to query specific information from it.

Accepted commands:
- NORNSCTL_COMMAND_PAUSE_ACCEPT: Stop accepting incoming tasks.
- NORNSCTL_COMMAND_RESUME_ACCEPT: Resume accepting incoming tasks after
    a successful NORNSCTL_COMMAND_PAUSE_ACCEPT has been received.
- NORNSCTL_COMMAND_PING: Check whether the server is up and running.

This new function allows removing the deprectated nornsctl_ping() function from
libnornsctl.so, since an equivalent functionality can now be supported using
the NORNSCTL_COMMAND_PING argument. Therefore, any such invocation:

    norns_error_t rv = nornsctl_ping();

can now be replicated as:

    norns_error_t rv = nornsctl_send_command(NORNSCTL_COMMAND_PING, NULL);

Closes #7

See merge request !2

This commit fixes #6.

Closes #5.

Setting the KEEP_TEST_SUBDIR environment variable to 1 before invoking
the API tests instructs the test infrastructure to not delete the test
subdir, even if the tests finished succesfully.

Added a new 'log_file' option to urd's configuration file that instructs
it to save all log messages to the configured file. As of now, this
option is only honored if 'use_syslog' is set to false.

This commit closes #4.

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.