Logger PMDA can now handle multiple clients.

Logger PMDA can now handle multiple clients.

* pcp/src/pmdas/logger/event.c (ctx_start_callback): New function.
  (ctx_end_callback): Ditto.
  (event_init): Register callbacks.
  (event_fetch): Call event_create to get new data.
* pcp/src/pmdas/logger/event.h: Removed event_create() prototype.
* pcp/src/pmdas/logger/logger.c (logger_end_contextCallBack): No longer
  call event_cleanup(), which is now called in ctx_end_callback().
* pcp/src/pmdas/logger/logger.h: Added get_monitor_fd() prototype.
* pcp/src/pmdas/logger/loggerMain.c (get_monitor_fd): New function.
  (loggerMain): Open the logfile with O_NONBLOCK.  Let event_fetch() call
  event_create().
* pcp/src/pmdas/logger/percontext.h: Update comment.