initdb initial password from file

Attached is a patch that adds the option --pwfile=<filename> to initdb.
The first line of this file is used to set the new superuser password
(the same way --pwprompt asks for one).

This feature is needed for the win32 GUI installer (possibly other
installers?), which need a way to specify the password to be set. Per
previous discussions commandline parameter or environment variable are
not secure. Storing it in a file is the same thing libpq does for
~/.pgpass, and should be secure enough, I think.

//Magnus