travis

* fix (Linux) agent build: run `make setagent` and `make all` first
* force travis to use clang compiler when configured to do so
* use travis-matrix's fast finish option
* delete run for windows_agent with clang (since we are cross-compiling)
* null-terminate buffer in `OS_RecvConnUDP`
  - this is not a severe issue, cause this function is not used beside the testing suite