BUG/MINOR: stats: correctly report throttle rate of low weight servers

The throttling of low weight servers (<16) could mistakenly be reported
as > 100% due to a rounding that was performed before a multiply by 100
instead of after. This was introduced in 1.5-dev20 when fixing a previous
reporting issue by commit d32c399 (MINOR: stats: report correct throttling
percentage for servers in slowstart).

It should be backported if the patch above is backported.
BUG/MEDIUM: checks: servers must not start in slowstart mode

In 1.5-dev20, commit bb9665e (BUG/MEDIUM: checks: ensure we can enable
a server after boot) tried to fix a side effect of having both regular
checks and agent checks condition the up state propagation to servers.

Unfortunately it was still not fine because after this fix, servers
which make use of slowstart start in this mode. We must not check
the agent's health if agent checks are not enabled, and likewise,
we must not check the regular check's health if they are not enabled.

Reading the code, it seems like we could avoid entering this function
at all if (s->state & SRV_RUNNING) is not satisfied. Let's reserve
this for a later patch if needed.

Thanks to Sander Klein for reporting this abnormal situation.