BUG/MEDIUM: pollers: really create the poller pipes in builds without threads

BUG/MEDIUM: pollers: really create the poller pipes in builds without threads

Commit c5f41118c ("BUG/MEDIUM: tasks: don't type-pun state and tid in
builds without threads") was mangled when committed: instead of the task
fix its message describes, its content accidentally reverted commit
cc0c18d82 ("BUG/MEDIUM: pollers: create the poller pipes in builds
without threads"), moving the fd_precreate_poller_pipes() call back into
setup_extra_threads(), an empty stub without threads. Builds made with
USE_THREAD unset (e.g. TARGET=generic) thus abort again on startup on
BUG_ON(fdtab[fd].owner) when inserting the poller pipes.

Move the call back to haproxy.c, right before setup_extra_threads(), so
that all builds share the same path.