Fix source_tree variable in dkms build

The source_tree variable in the previous commit had an extra $.
Remove it so that source_tree is expanded properly.  An identical
fix has been applied in the original patch to the stable branch.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2776
ztest: print backtrace on SIGSEGV and SIGABRT

Add signal handlers to print a backtrace if we crash or assert.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2788
This is iSCSI sharing for ZoL (shareiscsi).
* Supports the following iSCSI implementations (in this order of discovery):
  + IET         http://iscsitarget.sourceforge.net
  + STGT        http://stgt.berlios.de
  + SCST        http://scst.sourceforge.net
    + Requires that SCST was compiled with /sys support (the default).
  + LIO         http://linux-iscsi.org
* Will refuse to unshares an active target (one with sessions).
* Supports the following options to the 'shareiscsi' property:
  + name/iqn    Full iSCSI Qualified Name (IQN), including identifier.
                This is generated by iscsi_generate_target():
                + Uses the content of an optional /etc/iscsi_target_id file:
                  Example: iqn.YYYY-MM.tld.domain
                + If this file doesn't exist, it uses the current year, month
                  and domain name to generate the iqn.
                + The dataset name is appended at the end of the iqn, with
                  slashes replaced with dots.
                  => <iqn>:<dataset>
  + lun         LUN (0-16384)
                Default: 0 (1 for STGT)
  + type        Share mode (fileio, blockio, nullio, disk, tape)
                STGT: ssc, pt
                Default: blockio (disk for STGT)
  + iomode      IO mode (wb, wt, ro)
                STGT: rdwr, aio, mmap, sg, ssc
                Default: wt (rdwr for STGT)
  + blocksize   Logical block size (512, 1024, 2048, 4096)
                Default: Volume blocksize, 4096 if not usable.
                NOTE: Currently not supported for STGT (doesn't seem to be
                      an option for it in tgtadm).
  + initiator   Allow only this initiator to bind to target.
                Currently only availible for LIO, STGT and SCST.
  + authname    Global user to use in binds on targets.
  + authpass    Password for global user.
* If not called with a 'name/iqn' value, then force setting it.
  This so that the IQN doesn't change every month (when it is
  regenerated again). It will be generated by iscsi_generate_target()
  (see above).

NOTE:
+ Moved nfs.c:foreach_nfs_shareopt() to libshare.c:foreach_shareopt()
  so that it can be (re)used in smb.c and iscsi.c.
+ Split iSCSI implementation into their own separate source files
+ Use the list_{create,insert}() etc for keeping tabs of the linked lists
  instead of using a home-made version.
+ A half second delay had to be added in lib/libzfs/libzfs_mount.c:zfs_unmount()
  after the successful unshare. This to avoid 'dataset busy' when destroying
  recursivly.
= The 'initiator', 'authname' and 'authpass' option might have some issues:
  It will make the compatibility between the different iSCSI implementations
  questionable - can't switch between them easily (it will ONLY be availible
  in ZoL). I COULD make the option be silently ignored (instead of forcibly
  rejected if not availible).

  But that still doesn't solve the (possible) problem between ZoL and
  OpenZFS/Illumos (setting the option, will possibly introduce problems when
  importing the pool on something else than ZoL).

  So some more discussion might be needed..
Misc iSCSI fixes/cleanups/attempts to get 'zfs rename' to work.

!! Doesn't - a 'zfs rename' works, but most often than not, the rename
!! IOCTL hangs. Not every time and not always on the first dataset being
!! renamed... Seems a little to random to me.