Bugfixes

* Update two minor copyright statements [MCR]
* Make -lgmp into a variable [MCR]
* Clarify use of Makefile.vendor, and clean it out. [MCR]
* buildlin should warn if WERROR set, and look for missing gmp.h [MCR]
* Target build for Trusty by instead of Precise [Simon Deziel]
* Improve wording of leftsubnets' explanation in the man page [Simon Deziel]
* Bring back NAT traversal that got mistakenly pulled out by CVE-2014-2037 patch. [Thomas Geulig]