DOC: fix a few typos

include/types/proto_http.h: hwen -> when
include/types/server.h: SRV_ST_DOWN -> SRV_ST_STOPPED
src/backend.c: prefer-current-server -> prefer-last-server

Signed-off-by: Godbach <nylzhaowei@gmail.com>
(cherry picked from commit f2dd68d0e047cdd433d698f044bb2d327886fc66)
BUG/MEDIUM: sample: fix random number upper-bound

random() will generate a number between 0 and RAND_MAX. POSIX mandates
RAND_MAX to be at least 32767. GNU libc uses (1<<31 - 1) as
RAND_MAX.

In smp_fetch_rand(), a reduction is done with a multiply and shift to
avoid skewing the results. However, the shift was always 32 and hence
the numbers were not distributed uniformly in the specified range. We
fix that by dividing by RAND_MAX+1. gcc is smart enough to turn that
into a shift:

    0x000000000046ecc8 <+40>:    shr    $0x1f,%rax
(cherry picked from commit 1228dc0e7ae4a6b16c0c7f74a28f8e84601b526c)