[multipathd] switch on/off the queueing feature when oportune

[multipathd] switch on/off the queueing feature when oportune

The following patch adds time-based retry feature in no-path situation
to multipathd.  Any comments are welcome.

This patch adds 'no_path_retry' option to multipathd.

o no_path_retry = "fail" is equal to 'fail_if_no_path'.
  (i.e. I/O to the no-path map will immediately fail.)

o no_path_retry = "queue" is equal to 'queue_if_no_path'.
  (i.e. I/O to the no-path map will be queued until any path comes up.)

o If no_path_retry = <n> where n is positive number,
  then multipathd will set queue_if_no_path to the map,
  and if the all paths are down, multipathd will turn the feature
  off to fail_if_no_path after the checker tries <n> times for each
  paths in the map.

Multipathd re-writes queue_if_no_path feature parameter in the map,
if this option is specified.  But by default, this patch doesn't
change current multipathd behaviour.
So this patch don't break any existing configuration.

Kiyoshi Ueda, NEC