mail: improve temp file selection

[mknos]

• Use logic in File::Temp to avoid name collision for temporary file
• I tested this by loading a mailbox, selecting message 2 then "viewing" it...

perl mail # reads $HOME/mbox by default
2
v
...quit vi
q

• We don't need to set file with $tmbox->file(...) because new() already set it
• While here, remove useless debug message about entering "stuff" function