This effectively reverts 271a663d "rtl: support
start/stop threads around pre-init" again, now that
df6ba650 "Remove 'officially dead now' rtl_cache
slab allocator mechanism" removed the wsupdate thread.

(rtl_alloc_preInit is an internal-use-only C function, so changing its arguments
doesn't affect URE compatibility.)

Change-Id: Ie9bce86377f9520e2600e4111ac525dddace10f8
Reviewed-on: https://gerrit.libreoffice.org/58443
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>