Adding some msvc specific defines

Adding some msvc specific defines

Summary:
msvc puts ssize_t in a stupidly odd place and names it weirdly too
this also takes care of snprintf missing (the semantics are slightly off in the msvc version regarding the return value, but usage in folly is limited and does NOT do the double snprintf call madness so this is safe)
funcsig and pretty function give you roughtly the same thing in compiler specific implementations
strerror_s is msvc's thread save strerror

@override-unit-failures

Test Plan: fbmake runtests

Reviewed By: delong.j@fb.com

FB internal diff: D1291542