Most of the code in llog.c and llog_cat.c is completely unused
on the client and can be removed, as a preparation for removing
the dt_object code.
Two tricky parts are:
- In llog_cat_close(), we rely on the fact that llh_flags never
contains LLOG_F_ZAP_WHEN_EMPTY, because nobody ever sets that
flag.
- In llog_read_header(), we check the return value of the lpi_cb
callback, and again we know that it cannot be LLOG_PROC_BREAK
or LLOG_DEL_RECORD and can remove the respective code path.