BUG/MINOR: haload: split the rate exactly across threads

BUG/MINOR: haload: split the rate exactly across threads

hld_usr_schedule() and hld_rate_task() gave every thread ceil(rate /
arg_thrd), so a low rate on a machine with many threads was quietly
served several times over (e.g. rate=1 on 200 threads gave ~200 req/s
instead of 1). Use the same exact split already used elsewhere in
haload (thread <tid> gets (rate + tid) / arg_thrd): most threads then
get 0 and only <rate> of them get 1, so the total always matches
<rate> regardless of the thread count.

A thread with no budget now just checks back in 1s instead of being
bumped to 1 by the old max ? max : 1 fallback.