[PATCH] Log PostgreSQL version number on startup

Hi,

it has bugged me for a long time that there's no clear "PostgreSQL is
starting" message in the server log file. I'd like to change that for
two reasons:

* when reading a long log file, it's not entirely clear where a new
server startup begins.

It was worse in the past (first message was "database system was
shut down at..." which rather looks like a shutdown message to
newbie users), now it's "listening on IPv6..." but that message
might possibly become DEBUG1, or listen_addresses might become
changeable at run-time

* while upgrading servers, it's useful to know which server version
was actually started if there are several startup attempts

The change would be to log "Starting PG_VERSION_STR" as early as
possible.

A startup looks like this:

2018-11-21 15:19:47.259 CET [24453] LOG: listening on IPv6 address "::1", port 5431
2018-11-21 15:19:47.259 CET [24453] LOG: listening on IPv4 address "127.0.0.1", port 5431
2018-11-21 15:19:47.315 CET [24453] LOG: listening on Unix socket "/tmp/.s.PGSQL.5431"
2018-11-21 15:19:47.394 CET [24453] LOG: starting PostgreSQL 12devel on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.2.0-9) 8.2.0, 64-bit
2018-11-21 15:19:47.426 CET [24454] LOG: database system was shut down at 2018-11-21 15:15:35 CET
2018-11-21 15:19:47.460 CET [24453] LOG: database system is ready to accept connections

(I'd rather put the start message before the listening messages, but I
think the startup message should be logged via logging_collector, and
listening is logged before the log file is opened.)

Christoph
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