In initdb, don't bother trying max_connections = 10.

In initdb, don't bother trying max_connections = 10.

The server won't actually start with that setting anymore, not since
we raised the default max_wal_senders to 10.  Per discussion, we don't
wish to back down on that default, so instead raise the effective floor
for max_connections (to 20).  It's still possible to configure a smaller
setting manually, but initdb won't set it that way.

Since that change happened in v10, back-patch to v10.

Kyotaro Horiguchi

Discussion: https://postgr.es/m/20180209.170823.42008365.horiguchi.kyotaro@lab.ntt.co.jp