SHMEM_INDEX_SIZE exceeded on startup

I doubt that this is going to matter much, and should only have a
trivial impact on shared space calculations and postmaster and
connection startup time, but just as a matter of principle we might
want to set SHMEM_INDEX_SIZE at least as large as the number of
entries in ShmemIndex.  At startup that seems to be 40 as of current
HEAD.

Trivial patch attached.

-Kevin