bees: add -G/--thread-min option for minimum thread count

The -g option limits the number of worker threads when the target load
average is exceeded.  On some systems the load normally runs high, and
continuous bees operation is required to avoid running out of disk space.

Add a -G/--thread-min option to force at least some threads to continue
running.

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

include/crucible/task.h

@@ -58,6 +58,9 @@ namespace crucible {
 		// Blocks until the running thread count reaches this number
 		static void set_thread_count(size_t threads);
 
+		// Sets minimum thread count when load average tracking enabled
+		static void set_thread_min_count(size_t min_threads);
+
 		// Calls set_thread_count with default
 		static void set_thread_count();