This is iSCSI sharing for ZoL (shareiscsi).
* Supports the SCST and IET iSCSI implementations.
  + Requires that SCST was compiled with /sys support (does not work
    with SCST with /proc support - this is deprecated/not recomended
    anyway).
* Does not 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 todays 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
  + type	Share mode (fileio, blockio, nullio, disk, tape)
		Default: blockio
  + iomode	IO mode (wb, wt, ro)
		Default: wt
  + blocksize	Logical block size (512, 1024, 2048, 4096)
		Default: Volume blocksize, 4096 if not usable.
* 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.