Provide modern examples of how to auto-start Postgres on macOS.

The scripts in contrib/start-scripts/osx don't work at all on macOS
10.10 (Yosemite) or later, because they depend on SystemStarter which
Apple deprecated long ago and removed in 10.10.  Add a new subdirectory
contrib/start-scripts/macos with scripts that use the newer launchd
infrastructure.

Since this problem is independent of which Postgres version you're using,
back-patch to all supported branches.

Discussion: https://postgr.es/m/31338.1510763554@sss.pgh.pa.us

contrib/start-scripts/macos/README

+To make macOS automatically launch your PostgreSQL server at system start,
+do the following:
+
+1. Edit the postgres-wrapper.sh script and adjust the file path
+variables at its start to reflect where you have installed Postgres,
+if that's not /usr/local/pgsql.
+
+2. Copy the modified postgres-wrapper.sh script into some suitable
+installation directory.  It can be, but doesn't have to be, where
+you keep the Postgres executables themselves.
+
+3. Edit the org.postgresql.postgres.plist file and adjust its path
+for postgres-wrapper.sh to match what you did in step 2.  Also,
+if you plan to run the Postgres server under some user name other
+than "postgres", adjust the UserName parameter value for that.
+
+4. Copy the modified org.postgresql.postgres.plist file into
+/Library/LaunchDaemons/.  You must do this as root:
+    sudo cp org.postgresql.postgres.plist /Library/LaunchDaemons
+because the file will be ignored if it is not root-owned.
+
+At this point a reboot should launch the server.  But if you want
+to test it without rebooting, you can do
+    sudo launchctl load /Library/LaunchDaemons/org.postgresql.postgres.plist

contrib/start-scripts/macos/org.postgresql.postgres.plist

+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>Label</key>
+	<string>org.postgresql.postgres</string>
+	<key>ProgramArguments</key>
+	<array>
+		<string>/bin/sh</string>
+		<string>/usr/local/pgsql/bin/postgres-wrapper.sh</string>
+	</array>
+	<key>UserName</key>
+	<string>postgres</string>
+	<key>KeepAlive</key>
+	<true/>
+</dict>
+</plist>

contrib/start-scripts/macos/postgres-wrapper.sh

+#!/bin/sh
+
+# PostgreSQL server start script (launched by org.postgresql.postgres.plist)
+
+# edit these as needed:
+
+# directory containing postgres executable:
+PGBINDIR="/usr/local/pgsql/bin"
+# data directory:
+PGDATA="/usr/local/pgsql/data"
+# file to receive postmaster's initial log messages:
+PGLOGFILE="${PGDATA}/pgstart.log"
+
+# (it's recommendable to enable the Postgres logging_collector feature
+# so that PGLOGFILE doesn't grow without bound)
+
+
+# set umask to ensure PGLOGFILE is not created world-readable
+umask 077
+
+# wait for networking to be up (else server may not bind to desired ports)
+/usr/sbin/ipconfig waitall
+
+# and launch the server
+exec "$PGBINDIR"/postgres -D "$PGDATA" >>"$PGLOGFILE" 2>&1

contrib/start-scripts/osx/README

+The scripts in this directory are for use with Apple's SystemStarter
+infrastructure, which is deprecated since macOS 10.4 and is gone entirely
+as of 10.10.  You should use the scripts in ../macos instead, unless
+you are using a macOS release too old to have launchd.
+
 To install execute the following:
 
 sudo /bin/sh ./install.sh