Add zfs-helpers.sh script

Add a script designed to facilitate in-tree development and testing
by installing symlinks on your system which refer to in-tree helper
utilities.  These helper utilities must be installed to in order to
exercise all ZFS functionality.  By using symbolic links and keeping
the scripts in-tree during development they can be easily modified
and those changes tracked.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #4607

zfs-script-config.sh.in

@@ -19,9 +19,11 @@ export ZPIOSPROFILEDIR=${BUILDDIR}/scripts/zpios-profile
 export ETCDIR=${SRCDIR}/etc
 export TESTSDIR=${SRCDIR}/tests
 export RUNFILEDIR=${TESTSDIR}/runfiles
+export UDEVRULEDIR=${BUILDDIR}/udev/rules.d
 
 export ZDB=${CMDDIR}/zdb/zdb
 export ZFS=${CMDDIR}/zfs/zfs
+export ZHACK=${CMDDIR}/zhack/zhack
 export ZINJECT=${CMDDIR}/zinject/zinject
 export ZPOOL=${CMDDIR}/zpool/zpool
 export ZTEST=${CMDDIR}/ztest/ztest