Make improvements

- Polyfill readlink("foo/") dir check on Windows
- Support asynchronous signal delivery on Windows
- Restore Windows Console from execve() daisy chain
- Work around bug in AARCH64 Optimized Routines memcmp()
- Disable unbourne.com shell completion on Windows for now
- Don't always set virtual terminal input state on console
- Remove Musl Libc's unusual preservation of realpath("//")
- Make realpath() strongly link malloc() to pass configure test
- Delete cosh.com shell, now that unbourne.com works on Windows!

examples/unbourne.c

@@ -5816,8 +5816,11 @@ static ssize_t preadfd(void) {
 retry:
   if (!parsefile->fd && isatty(0)) {
     linenoiseSetFreeHintsCallback(free);
-    linenoiseSetHintsCallback(ShellHint);
-    linenoiseSetCompletionCallback(ShellCompletion);
+    if (!IsWindows()) {
+      // TODO(jart): Cache $PATH search.
+      linenoiseSetHintsCallback(ShellHint);
+      linenoiseSetCompletionCallback(ShellCompletion);
+    }
     if ((p = linenoiseWithHistory(">: ", "unbourne"))) {
       nr = min(strlen(p), IBUFSIZ - 2);
       memcpy(buf, p, nr);