Use RequiresMountsFor on datadir

It is quite common for the data directory to be on its own mount.
Systemd can ensure a directory is mounted before a service starts using
RequiresMountsFor[1]:

> Takes a space-separated list of absolute paths. Automatically adds
> dependencies of type Requires= and After= for all mount units required
> to access the specified path.

[1]: https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html#RequiresMountsFor=

Author: ekohl

templates/systemd-override.conf.epp

@@ -3,6 +3,9 @@
   Stdlib::Absolutepath             $datadir,
   Optional[String[1]]              $extra_systemd_config,
 | -%>
+[Unit]
+RequiresMountsFor=<%= $datadir %>
+
 [Service]
 Environment=PGPORT=<%= $port %>
 <%- if $facts['os']['family'] == 'Gentoo' { -%>