Thread: dsm.c API tweak

Per 
https://postgr.es/m/CAEepm=11ma_Z1HoPxPcSCANnh5ykHORa=HcA1U1V1+5S_jwPuA@mail.gmail.com
it seems that the dsm.c API is a bit inconvenient right now.  I proposed
in the first patch in that thread to change the API so that a segment is
marked as "pinned" if created with no ResourceOwner set as current;
which is essentially the same as creating a fake one, then marking the
segment as pinned.

Thomas agrees with me, so I propose this patch as preparatory work for
the autovacuum/BRIN stuff I'm interested in.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services