Method "-o keysource=passphrase,file://" was missing pkcs11_read_data(), lift
that in from OpenSolaris. Reading from file and /dev/random works hopefully
fixing https://github.com/zfsrogue/zfs-crypto/issues/20
* Retrive active IET sessions from /proc/net/iet/session
  + New struct iscsi_session_s.
  * New entry in struct iscsi_target_s: session which is a link to
    the corresponding iscsi_session_s entry for the target.
  + Use this where needed to make sure we don't enable/disable/modify
    a share with active sessions.
* Fail silently (with SA_SYSTEM_ERR, not -1) in iscsi_generate_target().
* Remove /proc/net/iet/volume example and move them to iscsi.h above
  the corresponding struct definition.
* Modified (and hopfully improved) debug messages.
* Improve share/unshare and force name=iqn.XXX option in iscsi_update_shareopts().
* Retrive active IET sessions from /sysfs.
* Add some debugging to iscsi_read_sysfs_value().