Remove our artificial PG_SOMAXCONN limit on listen queue length.

Remove our artificial PG_SOMAXCONN limit on listen queue length.

I added this in commit 153f40067, out of paranoia about kernels
possibly rejecting very large listen backlog requests.  However,
POSIX has said for decades that the kernel must silently reduce
any value it considers too large, and there's no evidence that
any current system doesn't obey that.  Let's just drop this limit
and save some complication.

While we're here, compute the request as twice MaxConnections not
twice MaxBackends; the latter no longer means what it did in 2001.

Per discussion of a report from Kevin McKibbin.

Discussion: https://postgr.es/m/CADc_NKg2d+oZY9mg4DdQdoUcGzN2kOYXBu-3--RW_hEe0tUV=g@mail.gmail.com