Increase the maximum initdb-configured value for shared_buffers to 128MB.

Increase the maximum initdb-configured value for shared_buffers to 128MB.

The old value of 32MB has been around for a very long time, and in the
meantime typical system memories have become vastly larger.  Also, now
that we no longer depend on being able to fit the entirety of our
shared memory segment into the system's limit on System V shared
memory, there's a much better chance of the higher limit actually
proving productive.

Per recent discussion on pgsql-hackers.