Change DEFAULT_UDEV_SYNC to 1 so udev_sync is used even without any config.

Change DEFAULT_UDEV_SYNC to 1 so udev_sync is used even without any config.

This should be set by default! Normally we have "activation/udev_sync = 1"
in lvm.conf (example.conf.in). But if we use lvm2 without any config file
(or without a definition within '--config' option) the DEFAULT_UDEV_SYNC
is used instead. Together with verify_udev_operations=0 (when we rely on
udev fully), this can cause races as the node could be missing when needed.

(See also https://bugzilla.redhat.com/show_bug.cgi?id=723144)