Refuse to package up files for publication if their permissions are set
such that they will not be readable by the web server.
Don't just print an error message, actually refuse to keep going.
Refuse to create a new package of bits with the same name as an old package.
The mirroring code keeps track of which packages it has downloaded already,
so this would result in bits never reaching the mirrors.

Submitted by:	delphij
Fix my previous commit here: It's the *staging* directory which needs to
have world-readable files in it, not our private state directory. [1]

While here, don't warn about the directory itself, since we only tar up
the contents to send out to the mirrors. [2]

Pointy hat to:	cperciva [1]
Submitted by:	des [2]