LU-15938 llog: llog_reader to detect more corruptions

LU-15938 llog: llog_reader to detect more corruptions

Improve llog_reader to determine more corruptions and report
errors
 - notify if llog bitmap has bits set with no records in llog
 - compare header records count with amount of records really
   found
 - fix amount of records to output, preventing wrong output of
   NOT SET record
 - list missing records in gap if found
 - count all errors found, add prefix 'error:' in output for
   better output processing by third-party scripts
 - don't exit immediately in case of error but continue if
   possible and output all read valid data

Lustre-change: https://review.whamcloud.com/47934
Lustre-commit: 74bb524ee9b437777ecfe4e99fd032d8cac95cd0

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ic47dc6bb6cbdd9db6f888a0b892254403a628912
Reviewed-on: https://review.whamcloud.com/47949
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>