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/postgres-wrapper.sh

@@ -0,0 +1,25 @@
+#!/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