src: add IsFunction() assert to MakeCallback

Helps catch bugs early on. Without it, V8 throws the fairly
unhelpful exception "TypeError: undefined is not a function" -
unhelpful because there is no stack trace.
src: ReqWrap now optionally accepts a wrap object
udp_wrap, tcp_wrap: add out arg to AddressToJS

Prep work for removing process._errno. The handle.getsockname() function
will return a status code in the future and set the address and port
properties on the object that's passed in from JS land.
src, lib: remove process._errno, step 1

C++ functions no longer set process._errno, they just return the error
code now. First pass, more to follow.