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!

libc/intrin/exit.c

@@ -85,10 +85,6 @@ wontreturn void _Exit(int exitcode) {
 #endif
   } else if (IsWindows()) {
     uint32_t waitstatus;
-    // Restoring the CMD.EXE program to its original state is critical.
-    if (_weaken(__restore_console_win32)) {
-      _weaken(__restore_console_win32)();
-    }
     // What Microsoft calls an exit code, POSIX calls a status code. See
     // also the WEXITSTATUS() and WIFEXITED() macros that POSIX defines.
     waitstatus = exitcode;