Obtain device list from udev by default if LVM2 is compiled with udev support.

Obtain device list from udev by default if LVM2 is compiled with udev support.

Also, add a new 'obtain_device_list_from_udev' setting to lvm.conf with which
we can turn this feature on or off if needed.

If set, the cache of block device nodes with all associated symlinks
will be constructed out of the existing udev database content.
This avoids using and opening any inapplicable non-block devices or
subdirectories found in the device directory. This setting is applied
to udev-managed device directory only, other directories will be scanned
fully. LVM2 needs to be compiled with udev support for this setting to
take effect. N.B. Any device node or symlink not managed by udev in
udev directory will be ignored with this setting on.