Call `udevadm trigger` before scanning devices

Device symlinks are setup by a usermode helper on Linux. It has been
observed that they can fall out of sync with actual devices. This has
the unpleasant effect of having devices be renamed. This patch is an
attempt to resolve that by invoking udevadm to generate the required
symlinks before scanning for devices on systems that use it.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Fork will return -1. If the binary running this has other children, it will block until one of them exits, which is undesirable. Otherwise, it will pretend nothing happened. I have revised the pull request to address this.