Improve attachment file handling: use one new function to create a temp

Improve attachment file handling: use one new function to create a temp
file for storing the attachment. This replaces the same code in five
places. It also improves on the code, it's now much more safe against
overwriting existing attachments by chance.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@12097 7612ce4b-ef26-0410-bec9-ea0150e637f0