Unix errno types, maps, and support

unix-errno provides an errno variant similar to Unix.error but including POSIX 2008 and Linux-specific constructors. A macro definition type, defns is also provided in order to transport a specific errno-integer map as is the case with FUSE. The types and their functions reside in Errno and are independent of any Unix bindings. This makes the library's types usable from MirageOS on top of Xen. Errno_unix provides maps to and from Unix.error, the present host's errno map, an errno exception Error, and higher-order errno checking functions.