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
Use dsl_dataset_snap_lookup()

Retire the dmu_snapshot_id() function which was introduced in the
initial .zfs control directory implementation.  There is already
an existing dsl_dataset_snap_lookup() which does exactly what we
need, and the dmu_snapshot_id() function as implemented is racy.

https://github.com/zfsonlinux/zfs/issues/1215#issuecomment-12579879

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1238