[Samba] permission bits clobbered

[Panko, Kevin]

A file is chmod 600.  It gets opened on Windows, and it gets changed to 644.
Now the secrets are exposed to all users.  This is bad!

This happens if the user does:

N:\> echo foobar > secretfile

But it does stay at chmod 600 if he does (append instead of truncate):

N:\> echo foobar >> secretfile

Why does this happen?
The "create mask" parameter is set to 644.  I do not think this should apply
to files that already exist, but only to files that are created.

Samba version is 2.2.8a.