rtapi/rt-preempt: use distinct RTAPI module id's in ULAPI

this got in the way of creating several HAL components per process
which happened to share the same module_id (pid) and hence could not
be exited separately.

use same allocation method as src/rtapi/xenomai.c
rtapi/simple_strtoul(): move to rtapi.h, make visible for UL builds only
components/lcd.c: fix warning about abs()