Retire zpool_id infrastructure

In the interest of maintaining only one udev helper to give vdevs
user friendly names, the zpool_id and zpool_layout infrastructure
is being retired.  They are superseded by vdev_id which incorporates
all the previous functionality.

Documentation for the new vdev_id(8) helper and its configuration
file, vdev_id.conf(5), can be found in their respective man pages.
Several useful example files are installed under /etc/zfs/.

  /etc/zfs/vdev_id.conf.alias.example
  /etc/zfs/vdev_id.conf.multipath.example
  /etc/zfs/vdev_id.conf.sas_direct.example
  /etc/zfs/vdev_id.conf.sas_switch.example

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #981

scripts/common.sh.in

@@ -41,7 +41,7 @@ udevruledir=@udevruledir@
 sysconfdir=@sysconfdir@
 
 ETCDIR=${ETCDIR:-/etc}
-DEVDIR=${DEVDIR:-/dev/disk/zpool}
+DEVDIR=${DEVDIR:-/dev/disk/by-vdev}
 ZPOOLDIR=${ZPOOLDIR:-${pkglibexecdir}/zpool-config}
 ZPIOSDIR=${ZPIOSDIR:-${pkglibexecdir}/zpios-test}
 ZPIOSPROFILEDIR=${ZPIOSPROFILEDIR:-${pkglibexecdir}/zpios-profile}
@@ -50,7 +50,6 @@ ZDB=${ZDB:-${sbindir}/zdb}
 ZFS=${ZFS:-${sbindir}/zfs}
 ZINJECT=${ZINJECT:-${sbindir}/zinject}
 ZPOOL=${ZPOOL:-${sbindir}/zpool}
-ZPOOL_ID=${ZPOOL_ID:-${bindir}/zpool_id}
 ZTEST=${ZTEST:-${sbindir}/ztest}
 ZPIOS=${ZPIOS:-${sbindir}/zpios}
 
@@ -525,7 +524,7 @@ udev_trigger() {
 
 #
 # The following udev helper functions assume that the provided
-# udev rules file will create a /dev/disk/zpool/<CHANNEL><RANK>
+# udev rules file will create a /dev/disk/by-vdev/<CHANNEL><RANK>
 # disk mapping.  In this mapping each CHANNEL is represented by
 # the letters a-z, and the RANK is represented by the numbers
 # 1-n.  A CHANNEL should identify a group of RANKS which are all