Support having multiple Unix-domain sockets per postmaster.

Replace unix_socket_directory with unix_socket_directories, which is a list
of socket directories, and adjust postmaster's code to allow zero or more
Unix-domain sockets to be created.

This is mostly a straightforward change, but since the Unix sockets ought
to be created after the TCP/IP sockets for safety reasons (better chance
of detecting a port number conflict), AddToDataDirLockFile needs to be
fixed to support out-of-order updates of data directory lockfile lines.
That's a change that had been foreseen to be necessary someday anyway.

Honza Horak, reviewed and revised by Tom Lane

doc/src/sgml/ref/postgres-ref.sgml

@@ -254,8 +254,14 @@ PostgreSQL documentation
        <para>
         Specifies the directory of the Unix-domain socket on which
         <command>postgres</command> is to listen for
-        connections from client applications.  The default is normally
-        <filename>/tmp</filename>, but can be changed at build time.
+        connections from client applications.  The value can also be a
+        comma-separated list of directories.  An empty value
+        specifies not listening on any Unix-domain sockets, in which case
+        only TCP/IP sockets can be used to connect to the server.
+        The default value is normally
+        <filename>/tmp</filename>, but that can be changed at build time.
+        Specifying this option is equivalent to setting the <xref
+        linkend="guc-unix-socket-directories"> configuration parameter.
        </para>
       </listitem>
      </varlistentry>