Improve code hygiene of kdb5_util dump helpers

Improve code hygiene of kdb5_util dump helpers

kdb5_util dump can very briefly leak a file handle if the ok file
cannot be locked, or if the existing dump file cannot be read.  Add a
cleanup handler to prep_ok_file() and use proper output parameter
handling.  Change current_dump_sno_in_ulog() to close its file handle
before checking the result of fgets().  Reported by Bean Zhang.