LU-11264 llapi: clean up llapi_search_tgt() code

LU-11264 llapi: clean up llapi_search_tgt() code

Clean up llapi_search_tgt() and helper functions llapi_search_ost()
and llapi_search_mdt() to set errno on return.

Add man pages for all of these functions.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ieb2e93208fbc1b1492f632d8ce1383ca9fdec5f2
Reviewed-on: https://review.whamcloud.com/35092
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>