Fix potential file ownership conflict in mongooseim script

In the situation where two users (`user1`, `user2`) on the same machine try to use the mongooseim script, the one who tries to launch second will receive the following error:

    [user2@multiuser-machine MongooseIM]$ ./rel/mongooseim/bin/mongooseim start
    mkdir: cannot create directory `/tmp//home/user2': Permission denied

This commit changes the PIPE_DIR temporary directory to a flat one, thereby avoiding conflicts on `/tmp/home`