mail-default-directory / autosave

[David Reitter]

I was wondering if something can be done to get rid of auto-save  
files stored in the user's home directory whenever a *mail* buffer is  
created.
These auto-save files aren't deleted reliably (when the buffer is  
killed), unfortunately.,

`mail-default-directory' defaults to "~/". I thought about letting it  
default to `temporary-file-directory', but then you're obviously  
running into privacy issues and a file-name conflict.
On Darwin, there are directories /tmp/<user-id>, which are usually  
used for such business, but I guess this a system-specific standard.  
Does it make sense to introduce a system-specific default for this  
variable, or is there something better one could do?