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>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2056
Remove incorrect use of EXTRA_DIST for man pages

Setting the 'dist_' prefix is the correct way to instruct Automake
to include these files in the distribution.  The EXTRA_DIST variable
is reserved for files which are not covered by the automatic rules.

  http://www.gnu.org/software/automake/manual/automake.html#Basics

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