Bump soft open file limit (RLIMIT_NOFILE) to hard limit on startup

The default open file limit of 1024 is extremely low, given modern
resources and kernel architectures. The reason that this hasn't changed
change is because doing so would break legacy programs that use the
select(2) system call in hard to debug ways. So instead programs that
want to opt-in to a higher open file limit are expected to bump their
soft limit to their hard limit on startup. Details on this are very well
explained in a blogpost by the systemd author[1]. There's also a similar
change done by the Go language[2].

So this starts bumping postmaster and pgbench its soft open file limit
to the hard open file limit. Doing so is especially useful for the AIO
work that Andres is doing, because io_uring consumes a lot of file
descriptors. But even without looking at AIO there is a large number of
reports from people that require changing their soft file limit before
starting postgres, sometimes falling back to lowering
max_files_per_process when they fail to do so[3-8]. It's also not all
that strange to fail at setting the soft open file limit because there
are multiple places where one can configure such limits and usually only
one of them is effective (which one depends on how Postgres is started).

[1]: https://0pointer.net/blog/file-descriptor-limits.html
[2]: https://github.com/golang/go/issues/46279
[3]: https://serverfault.com/questions/785330/getting-too-many-open-files-error-for-postgres
[4]: https://serverfault.com/questions/716982/how-to-raise-max-no-of-file-descriptors-for-daemons-running-on-debian-jessie
[5]: https://www.postgresql.org/message-id/flat/CAKtc8vXh7NvP_qWj8EqqorPY97bvxSaX3h5u7a9PptRFHW5x7g%40mail.gmail.com
[6]: https://www.postgresql.org/message-id/flat/113ce31b0908120955w77029099i7ececc053084095a%40mail.gmail.com
[7]: https://github.com/abiosoft/colima/discussions/836
[8]: https://www.postgresql.org/message-id/flat/29663.1007738957%40sss.pgh.pa.us#2079ec9e2d8b251593812a3711bfe9e9