Update chat/prosody to 0.9.10.

Security
- mod_dialback: Adopt key generation algorithm from XEP-0185,
  to prevent impersonation attacks (CVE-2016-0756)

Fixes and improvements
- Startup: Open /dev/urandom read-only, to fix a failure to start
  on some systems (fixes #585)
- Networking: Improve handling of the 'select' network backend
  running out of file descriptors

Minor changes
- Networking: Increase default internal read size to prevent
  connections stalling with LuaEvent (see #583)
- DNS: Discard queries that failed to send due to connection
  errors (fixes #598)
- c2s, s2s: Lower priority of shutdown handler, so that modules
  such as MUC can always send shutdown notifications to (remote)
  users (fixes #601)

(fhajny)