initdb could use some lower default settings (trivial patch)

Installing postgres on a Solaris 8 system for personal use, I discovered that
initdb exceeded the shared memory available on my system (Default settings).
There's enough to run the database, but initdb doesn't probe to low enough
values:

for nconns in 100 50 40 30 20 10
for nbuffers in 1000 900 800 700 600 500 400 300 200 100 50

10/50 is too large, the largest Solaris can handle without changing the
defaults is 10/46. I think it'd be a good idea if initdb probed at least a
little lower here, say to a buffer size of 25, to let it install on Solaris
with default shmem settings.

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