build: squelch -Wdollar-in-identifier-extension warnings

The dtrace probes contain dollar signs. We know, llvm-gcc, and we don't
care. Suppress the warnings.
windows, unix: remove dead code
inet: snprintf returns int, not size_t
win: refactor uv_cpu_info

Fixes a couple of error handling issues:

* Don't free an uninitialized pointer when allocating memory for
  `cpu_infos` fails.
* Don't return a bogus error value when NtQuerySystemInformation fails.
  That function returns an NTSTATUS code instead of setting the last
  error.
* Don't clobber out parameters when an error happens.
build: add support for Visual Studio 2012

Closes #722
Merge branch 'v0.10'