Add detached node functions to ilist
These allow to test whether an element is in a list by checking whether
prev/next are NULL. Needed to replace SHMQueueIsDetached() when converting
from SHM_QUEUE to ilist.h style lists.
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/
20221120055930.t6kl3tyivzhlrzu2@awork3.anarazel.de
Discussion: https://postgr.es/m/
20200211042229.msv23badgqljrdg2@alap3.anarazel.de