sexycat: prepare to be used by sexywrap

  * add sexywrap support; this will enable the not-yet-committed sexywrap
    library to use parts of sexycat
  * add the -x flag (for sexywrap)
  * made a few functions (x*poll(), xpritev(), run_iscsi_event_loop())
    not to die on their own becasue they'll be used by sexywrap
  * improved user and program documentation
  * and a lot more
sexycat: add -I option to specify initiator name

...for the destinatio endpoint