fix linux

* error.BadFd is not a valid error code. it would always be a bug to
   get this error code.
 * merge error.Io with existing error.InputOutput
 * merge error.PathNotFound with existing error.FileNotFound.
   Not all OS's support both.
 * add os.File.openReadC
 * add error.BadPathName for windows file operations with invalid
   characters
 * add os.toPosixPath to help stack allocate a null terminating byte
 * add some TODOs for other functions to investigate removing the
   allocator requirement
 * optimize some implementations to use the alternate functions when
   a null byte is already available
 * add a missing error.SkipZigTest
 * os.selfExePath uses a non-allocating API
 * os.selfExeDirPath uses a non-allocating API
 * os.path.real uses a non-allocating API
 * add os.path.realAlloc and os.path.realC
 * convert many windows syscalls to use the W versions (See #534)

example/cat/main.zig

@@ -20,7 +20,7 @@ pub fn main() !void {
         } else if (arg[0] == '-') {
             return usage(exe);
         } else {
-            var file = os.File.openRead(allocator, arg) catch |err| {
+            var file = os.File.openRead(arg) catch |err| {
                 warn("Unable to open file: {}\n", @errorName(err));
                 return err;
             };