unlink old database file before creating a new one

unlink old database file before creating a new one

udev_db_add_device() can be called when the corressponding database
entry already exists - it should overwrite the old entry in this case.
However, if the old entry was a symlink, fopen(filename, "w") will not
overwrite it properly - it will keep the symlink and create a file
named after the symlink target.  Calling unlink(filename) before
trying to create the database file fixes the problem.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>