vdev_id: support per-channel slot mappings

The vdev_id udev helper currently applies slot renumbering rules to
every channel (JBOD) in the system.  This is too inflexible for systems
with non-homogeneous storage topologies.  The "slot" keyword now takes
an optional third parameter which names a channel to which the mapping
will apply.  If the third parameter is omitted then the rule applies to
all channels.  The first-specified rule that can match a slot takes
precedence.  Therefore a channel-specific rule for a given slot should
generally appear before a generic rule for the same slot number.  In
this way a custom slot mapping can be applied to a particular channel
and a default mapping applied to the rest.

Signed-off-by: Ned Bass <bass6@llnl.gov>