backend

I've completed the patch to fix the protocol and authentication issues I
was discussing a couple of weeks ago.  The particular changes are:

- the protocol has a version number
- network byte order is used throughout
- the pg_hba.conf file is used to specify what method is used to
  authenticate a frontend (either password, ident, trust, reject, krb4
  or krb5)
- support for multiplexed backends is removed
- appropriate changes to man pages
- the -a switch to many programs to specify an authentication service
  no longer has any effect
- the libpq.so version number has changed to 1.1

The new backend still supports the old protocol so old interfaces won't
break.