set passwd for users

Howdy:

Running PostgreSQL 7.2.1 on RedHat Linux 7.2.

Is it possible to have a password set for users for
command line use?

For example, I have scripts where I create and maintain
some aspects of the database. Let's say that a user
doesn't like the permission that I set for the table and wants
to change it themselves. They could do something like this:

testdb> \c database newuser

and are now the new user 'newuser'. How can I set
this up to work via the command line as well as with
some ODBC connection?

I am looking in the pg_hba.conf file and I see where
it talks about the system, permissions and such,
but I am more curious as to how to set my file
up so that maybe only the postgreSQL user can have
root-like accesses.

Thanks!

-X