Clients can now provide a 'track' flag when defining a namespace to let
urd know that it must keep track of the namespace contents. The flag is
currently propagated by the messaging system and stored in the internal
backend metadata. Tracking not implemented yet.

Resolve "Add support for deletion tasks"

Closes #8

See merge request !3

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.