pgsql: 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

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6313995c27f915333cc3c7aad521afda22a7ce48

Modified Files
--------------
contrib/start-scripts/macos/README                 | 24 +++++++++++++++++++++
.../macos/org.postgresql.postgres.plist            | 17 +++++++++++++++
contrib/start-scripts/macos/postgres-wrapper.sh    | 25 ++++++++++++++++++++++
contrib/start-scripts/osx/README                   |  5 +++++
4 files changed, 71 insertions(+)