BUG/MINOR: init: fix set-dumpable when using uid/gid

BUG/MINOR: init: fix set-dumpable when using uid/gid

in mworker mode used with uid/gid settings, it was not possible to get
a coredump despite the set-dumpable option.
indeed prctl(2) manual page specifies the dumpable attribute is reverted
to `/proc/sys/fs/suid_dumpable` in a few conditions such as process
effective user and group are changed.

this patch moves the whole set-dumpable logic before the polling code in
order to catch all possible cases where we could have changed the
uid/gid. It however does not cover the possible segfault at startup.

this patch should be backported in 2.0.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>