bees: dynamic thread pool size based on system load average

Add -g / --loadavg-target parameter to track system load and add or
remove bees worker threads dynamically to keep system load close to the
loadavg target.  Thread count may vary from zero to the maximum
specified by -c or -C, and is adjusted every 5 seconds.

This is better than implementing a similar load average scheme from
outside of the process (though that is still possible) because the
in-process load tracker does not disrupt the performance timing feedback
mechanisms as a freezer cgroup or SIGSTOP would when controlling bees
from outside.  The internal load average tracker can also adjust the
number of active threads while an external tracker can only choose from
the maximum or zero.

Also fix a bug where a Task could deadlock waiting for itself to exit
if it tries to insert a new Task after the number of worker threads has
been set to zero.

Also correct usage message for --scan-mode (values are 0..2) since
we are touching adjacent lines anyway.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/process.h

@@ -74,5 +74,8 @@ namespace crucible {
 	typedef ResourceHandle<Process::id, Process> Pid;
 
 	pid_t gettid();
+	double getloadavg1();
+	double getloadavg5();
+	double getloadavg15();
 }
 #endif // CRUCIBLE_PROCESS_H

include/crucible/task.h

@@ -61,6 +61,9 @@ namespace crucible {
 		// Calls set_thread_count with default
 		static void set_thread_count();
 
+		// Creates thread to track load average and adjust thread count dynamically
+		static void set_loadavg_target(double target);
+
 		// Writes the current non-executing Task queue
 		static ostream & print_queue(ostream &);