1) add support for joinable threads to Thread class;
2) move checking of thread model to Manager from mysqlmanager.cc,
because it is needed only for IM-main process.


server-tools/instance-manager/instance.cc:
  Use Manager::is_linux_threads() instead of global variable.
server-tools/instance-manager/listener.cc:
  Use Thread::start(DETACHED) instead of Thread::start_detached().
server-tools/instance-manager/manager.cc:
  1. Use Thread::start(DETACHED) instead of Thread::start_detached();
  2. Move checking of thread model to Manager from mysqlmanager.cc,
  because it is needed only for IM-main process.
server-tools/instance-manager/manager.h:
  Move checking of thread model to Manager from mysqlmanager.cc,
  because it is needed only for IM-main process.
server-tools/instance-manager/mysqlmanager.cc:
  Move checking of thread model to Manager from mysqlmanager.cc,
  because it is needed only for IM-main process.
server-tools/instance-manager/priv.cc:
  Move checking of thread model to Manager from mysqlmanager.cc,
  because it is needed only for IM-main process.
server-tools/instance-manager/priv.h:
  Move checking of thread model to Manager from mysqlmanager.cc,
  because it is needed only for IM-main process.
server-tools/instance-manager/thread_registry.cc:
  Add support of joinable threads to Thread class.
server-tools/instance-manager/thread_registry.h:
  Add support of joinable threads to Thread class.