adding PGPASSWORDFILE to libpq

Hello:

I'm giving a try at some TODO items. Currently it's the turn of the
PGPASSWORDFILE environment variable. I have modified libpq to make use
of this variable. I present the first cut here.

I have some questions:
- should there be some reasonable default non-empty value? If so, how
can I define it? In terms of $HOME?

- should there be a new parameter in the connection string that allows
specifying the file that should be used? A new parameter to
PQsetdbLogin() (breaks old code, I think) ?

- Should I try to use the passwd parameter as password file, and try to
use it as password if it fails to fopen()?

- Should the password be dependent of the database name?

Currently the format for the file should be

host:port:database:user:password

Any [other] comment on how this is done is welcome.

--
Alvaro Herrera (<alvherre[a]atentus.com>)
A male gynecologist is like an auto mechanic who never owned a car.
- Carrie Snow