Stability improvements.

Added checks for what needs to exist on (both) remote and local machine for
s3.xpra.*.sh to work.

Change-Id: I5e00fad54ab1b190697a6ab15338516f7471a8ca
Display and session name deduction fixed

* Error in display calculation fixed
* Session name deduction improved. Can never have worked. Possibly xpra
  server change caused this. Besides failing it was very slow. Current
  solution is unique enough, yet quite fast.

Change-Id: I30ce9e0f0ac61dd57bffed29c6ada71bf4dde973