dsm: allow use in single user mode.
It might seem pointless to allow use of dsm in single user mode, but otherwise
subsystems might need dedicated single user mode code paths.
Besides changing the assert, all that's needed is to make some windows code
assuming the presence of postmaster conditional.
Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA+hUKGL9hY_VY=+oUK+Gc1iSRx-Ls5qeYJ6q=dQVZnT3R63Taw@mail.gmail.com