REORG: backend: simplify conn_backend_get

REORG: backend: simplify conn_backend_get

Reorganize the conditions for the reuse of idle/safe connections :
- reduce code by using variable to store reuse mode and idle/safe conns
  counts
- consider that idle/safe/avail lists are properly allocated if
  max_idle_conns not null. An allocation failure prevents haproxy
  startup.

(cherry picked from commit 7f68d815af356fbe1b2e1080a88b9935581c91d2)
[wt: harmless, backported since needed for the next one after careful
     review]
Signed-off-by: Willy Tarreau <w@1wt.eu>