usbmount: stop using blkid, use udev environment variables instead

The blkid binary is not any longer installed when libblkid from
util-linux is selected. udev has a builtin blkid command though
and already detects all values that we are interested in. Change
the usbmount script to use the environment variables set by udev
instead of calling blkid directly.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>