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

src/backend/tcop/postgres.c

@@ -3344,7 +3344,7 @@ process_postgres_switches(int argc, char *argv[], GucContext ctx)
 				break;
 
 			case 'k':
-				SetConfigOption("unix_socket_directory", optarg, ctx, gucsource);
+				SetConfigOption("unix_socket_directories", optarg, ctx, gucsource);
 				break;
 
 			case 'l':