tell cmake that there is no library for dlopen on emscripten (shuts up warnings "emcc: cannot find library "dl"" for projects that add CMAKE_DL_LIBS to the link line)
Fix returning uninitialised memory to C code. flowinfo is a dead field that's unused, scope_id is actually useful but browsers have terrible support for link-local IPv6 addresses so we needn't support it now.
Merge pull request #2759 from NWilson/incoming

fix linker warning and a small bit of uninitialised memory
[tracing] Initial tracing API.

This is used in conjunction with a collection & analysis server
to analyze memory usage and other aspects of a program running
under emscripten.
[tracing] Integrate the tracing stuff.
[tracing] Instrument dlmalloc.c

These changes compile out entirely if tracing is not enabled.
[tracing] Initial trace collection server.