support sysfs entries.
fix: proto link error.
support exec_path_on_error parameter to invoke userland executable to notify.
a bit modify comments.
add several sysfs attributes.
refactor: splitted out wdev_util.{c,h} and wdev_ioctl.{c,h} from walb.c.
refactor: splitted out overlapped_io.{c,h} and pending_io.{c,h} from io.c.
add is_error_before_overflow parameter.
implement ioctl_wdev_search_lsid().
remove ioctl WALB_IOCTL_SEARCH_LSID.
walbctl: add search_valid_lsid command.
refactor: use check_and_open_bdev().
remove deprecated code.