Call 'udevadm settle' after operations which add/remove device nodes.

Call 'udevadm settle' after operations which add/remove device nodes.

Because udev operates asynchronously, we found errors which were
caused by a previous command (eg. sfdisk or pvremove) creating
or removing a device, and that change not having happened by the
time the next command was run.

This patch adds calls to '/sbin/udevadm settle' after any commands
which can add or remove device nodes.

If udev is not being used or not available, this should have no
effect.  The command fails and this is silently ignored.