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/runtime.sgml

@@ -1718,7 +1718,7 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
   <para>
    The simplest way to prevent spoofing for <literal>local</>
    connections is to use a Unix domain socket directory (<xref
-   linkend="guc-unix-socket-directory">) that has write permission only
+   linkend="guc-unix-socket-directories">) that has write permission only
    for a trusted local user.  This prevents a malicious user from creating
    their own socket file in that directory.  If you are concerned that
    some applications might still reference <filename>/tmp</> for the