[BUG] option allbackups was not working anymore in roundrobin mode

[BUG] option allbackups was not working anymore in roundrobin mode

Commit 3168223a7b33a1d5aad1e11b8f2ad917645d7f27 broke option
"allbackups" in roundrobin mode due to an erroneous structure
member replacement in backend.c. The PR_O_USE_ALL_BK flag was
not tested in the right member anymore.

This bug uncoverred another one, by which all backup servers would
be used whatever the option's value, if all of them had been seen
as simultaneously failed at one moment.

This patch fixes the two stupid errors. Correctness has been tested
using the test-fwrr.cfg config example.
(cherry picked from commit f4cca45b5e6c6ed88a0062cf92ae57e01405ab12)