First phase of FE/BE protocol modifications: new StartupPacket layout

with variable-width fields.  No more truncation of long user names.
Also, libpq can now send its environment-variable-driven SET commands
as part of the startup packet, saving round trips to server.

src/test/regress/expected/interval.out

@@ -1,7 +1,7 @@
 --
 -- INTERVAL
 --
-SET DATESTYLE = DEFAULT;
+SET DATESTYLE = 'ISO';
 -- check acceptance of "time zone style"
 SELECT INTERVAL '01:00' AS "One hour";
  One hour 

src/test/regress/sql/interval.sql

@@ -2,7 +2,7 @@
 -- INTERVAL
 --
 
-SET DATESTYLE = DEFAULT;
+SET DATESTYLE = 'ISO';
 
 -- check acceptance of "time zone style"
 SELECT INTERVAL '01:00' AS "One hour";