dropbear 2015.68 -> 2015.69

Known changes:
- Fix crash when forwarded TCP connections fail to connect
  (bug introduced in 2015.68)
- Avoid hang on session close when multiple sessions are started,
  affects Qt Creator
- Reduce per-channel memory consumption in common case, increase default
  channel limit from 100 to 1000 which should improve SOCKS forwarding
  for modern webpages
- Handle multiple command line arguments in a single flag
- Manpage improvements
- Build fixes for Android
- Don't display the MOTD when an explicit command is run
- Check curve25519 shared secret isn't zero