(Bug 5079) convert default email post security to usemask when needed

If user's default email post security was set to anything other than
public or private, and they posted by email with no security header,
DW would fail to convert it to usemask when storing attached images.
This patch runs the default setting through the same conversion as
the security header.
Merge pull request #407 from alierak/bug5079-emailpost-friends

(Bug 5079) convert default email post security to usemask when needed