pgsql: Add detached node functions to ilist

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/51384cc40c4a72c81d2475089ab8a1cf98ea56ff

Modified Files
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src/include/lib/ilist.h | 63 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 63 insertions(+)