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

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