libmount: read from udev, add --disable-libmount-udev-support

libmount: read from udev, add --disable-libmount-udev-support

The library traditionally uses libblkid to obtain device properties
(such as FS-type if not specified). This can be a relatively costly
operation to scan the device and requires read access to the device.

All relevant libblkid information is usually cached by the udev DB.
This commit adds the possibility to reuse the information from udev,
with a fallback to libblkid if udev is not available.

The commit also adds

 $ ./configure --disable-libmount-udev-support
 $ meson setup build -Dbuild-libmount-udev-support=disabled

to completely disable this feature and avoid libmount's dependence
on libsystemd.

Signed-off-by: Karel Zak <kzak@redhat.com>