postgresql: move caveats to commands.

Mirrors the `mysql` formula. Instead of telling people to type commands
actually run them for them instead.

Also, use the shorter, foreground manual run.

Closes Homebrew/homebrew#24437.

Formula/postgresql.rb

@@ -68,12 +68,14 @@ class Postgresql < Formula
     system "make install-world"
   end
 
+  def post_install
+    unless File.exist? "#{var}/postgres"
+      system "#{bin}/initdb", "#{var}/postgres", '-E', 'utf8'
+    end
+  end
+
   def caveats
     s = <<-EOS.undent
-    initdb #{var}/postgres -E utf8    # create a database cluster
-    postgres -D #{var}/postgres       # serve that database
-    PGDATA=#{var}/postgres postgres   # …alternatively
-
     If builds of PostgreSQL 9 are failing and you have version 8.x installed,
     you may need to remove the previous version first. See:
       https://github.com/mxcl/homebrew/issues/issue/2510
@@ -94,7 +96,7 @@ class Postgresql < Formula
     EOS
   end
 
-  plist_options :manual => "pg_ctl -D #{HOMEBREW_PREFIX}/var/postgres -l #{HOMEBREW_PREFIX}/var/postgres/server.log start"
+  plist_options :manual => "postgres -D #{HOMEBREW_PREFIX}/var/postgres"
 
   def plist; <<-EOS.undent
     <?xml version="1.0" encoding="UTF-8"?>