VERSION 1.1.2: bug fixes and NAT-PMP support

ZeroTier One version 1.1.2 is mostly a bug fix release, but also includes support
for NAT-PMP as a method of mapping external ports from behind supported routers.

Bugs fixed include:

 - Refactored path selection code to eliminate a change that might have introduced
   a reliability regression from earlier versions.
 - Reworked UPnP code for improved reliability and also added NAT-PMP support.
 - Fixed a crash on network preferred relays. (Not exploitable or security critical
   since only network controllers can send that config option and there was no
   buffer overflow or other exploitable issue.)
 - Fix GUI problem on OSX where window could be larger than screen on some Macbooks.
 - Fix a potential rate limit problem on PUSH_DIRECT_PATHS that could negatively
   impact same-network connectivity.
 - Update zerotier-cli to show preferred vs. active paths and hide inactive ones.
 - Fix an issue on shutdown with thread delete order and thread synchronization.