evdev: Support device hotplug with udev

This watches for new input devices and opens any that appear for event
dispatching. This is only supported when use_udev==1.

Review URL: https://codereview.chromium.org/150633005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248560 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248560