fssh: Don't lookup listening ports

* Looking up netstat ports requires a cross-platform way to use netstat
  and currently Linux and OS X don't have any obvious shared arguments.
  Lets try avoid the issue all together.
* Instead just assume that the RANDOM port is good, statisitcally it's
  very likely to be good unless connecting to a server that's trying to
  solve the C10k problem or something crazy.
* If the port is used, ask ssh to exit on port forwarding failure
  so the user knows.  Shouldn't be a big deal since fssh is intended
  to be used interactively.
ui_copy: fork ssh when writing copy-paste buffer

* Utilities such as xclip will block (actually forks into the
  background, but ssh waits for it terminate anyways) for as long
  as the copy-paste buffer is in use (basically until another process
  copies something).  Fork the ssh process so ui_copy returns
  immediately.