Unix errno types, maps, and support

unix-errno can be used with or without ctypes and OCaml's Unix module. Without ctypes and Unix, the basic types and functions are provided as well as Errno_host containing errno maps for popular operating systems. The errno-srcgen tool for generating OCaml source representing Errno.Host.t values will also be built. With ctypes and Unix, you'll also receive the errno-map tool for outputting the current host's errno map and the Errno_unix module containing an errno global variable checking function and Unix.error type converters.