LU-1302 llog: introduce llog_open

LU-1302 llog: introduce llog_open

- llog_open is pair to llog_close. Llog handle is allocated inside
  llog_open while llog_free_handle is part of llog_close. Each llog
  even non-existent should be opened and closed.
- llog_exist() is added to API to check if llog file exists
- llog_cat_put is renamed to llog_cat_close with cleaning up empty
  llogs like llog_obd_origin_cleanup did.

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: I6360b4c089ec1dde50289563447eefd3dfa4365a
Reviewed-on: http://review.whamcloud.com/3740
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>