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>

lib/process.cc

@@ -3,6 +3,7 @@
 #include "crucible/chatter.h"
 #include "crucible/error.h"
 
+#include <cstdlib>
 #include <utility>
 
 // for gettid()
@@ -118,4 +119,37 @@ namespace crucible {
 		return syscall(SYS_gettid);
 	}
 
+	double
+	getloadavg1()
+	{
+		double loadavg[1];
+		const int rv = ::getloadavg(loadavg, 1);
+		if (rv != 1) {
+			THROW_ERRNO("getloadavg(..., 1)");
+		}
+		return loadavg[0];
+	}
+
+	double
+	getloadavg5()
+	{
+		double loadavg[2];
+		const int rv = ::getloadavg(loadavg, 2);
+		if (rv != 2) {
+			THROW_ERRNO("getloadavg(..., 2)");
+		}
+		return loadavg[1];
+	}
+
+	double
+	getloadavg15()
+	{
+		double loadavg[3];
+		const int rv = ::getloadavg(loadavg, 3);
+		if (rv != 3) {
+			THROW_ERRNO("getloadavg(..., 3)");
+		}
+		return loadavg[2];
+	}
+
 }