vdev_id: improve keyword parsing flexibility

vdev_id: improve keyword parsing flexibility

The vdev_id udev helper strictly requires configuration file keywords
to always be anchored at the beginning of the line and to be followed
by a space character.  However, users may prefer to use indentation or
tab delimitation.  Improve flexibility by simply requiring a keyword
to be the first field on the line.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1239