Fix autoconf variable substitution in udev rules.

Change the variable substitution in the udev rule templates
according to the method described in the Autoconf manual;
Chapter 4.7.2: Installation Directory Variables.

The udev rules are improperly generated if the bindir parameter
overrides the prefix parameter during configure. For example:

1. ./configure --prefix=/usr/local --bindir=/opt/zfs/bin

The udev helper is installed as /opt/zfs/bin/zpool_id, but the
corresponding udev rule has a different path:

1. /usr/local/etc/udev/rules.d/60-zpool.rules ENV{DEVTYPE}=="disk", IMPORT{program}="/usr/local/bin/zpool_id -d %p"

The @bindir@ variable expands to "${exec_prefix}/bin", so it cannot
be used instead of @prefix@ directly.

This also applies to the zvol_id helper.

Closes #283.