Subject: [HACKERS] password authentication

This patch adds support for plaintext password authentication.  To use
it, you add a line like

host         all         0.0.0.0       0.0.0.0           password  pg_pwd.conf


to your pg_hba.conf, where 'pg_pwd.conf' is the name of a file containing
the usernames and password hashes in the format of the first two fields
of a Unix /etc/passwd file.  (Of course, you can use a specific database
name or IP instead.)

Then, to connect with a password through libpq, you use the PQconnectdb()
function, specifying the "password=" tag in the connect string and also
adding the tag "authtype=password".

I also added a command-line switch '-u' to psql that tells it to prompt
for a username and password and use password authentication.