Make improvements for Actually Portable Emacs

- Get SIGWINCH working again on the New Technology
- Correctly handle O_NOFOLLOW in open() on Windows
- Implement synthetic umask() functionality on Windows
- Do a better job managing file execute access on Windows
- Fill in `st_uid` and `st_gid` with username hash on Windows
- Munge UNICODE control pictures into control codes on Windows
- Do a better job ensuring Windows console settings are restored
- Introduce KPRINTF_LOG environment variable to log kprintf to a file

libc/calls/umask.c

@@ -17,14 +17,22 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
+#include "libc/intrin/atomic.h"
 #include "libc/intrin/strace.internal.h"
 #include "libc/sysv/consts/nr.h"
 
 /**
  * Sets file mode creation mask.
  *
+ * On Windows, the value of umask() determines how Cosmopolitan goes
+ * about creating synthetic `st_mode` bits. The default umask is 077
+ * which is based on the assumption that Windows is being used for a
+ * single user. Don't assume that Cosmopolitan Libc will help you to
+ * secure a multitenant Windows computer.
+ *
  * @return previous mask
  * @note always succeeds
  */
@@ -33,8 +41,7 @@ unsigned umask(unsigned newmask) {
   if (!IsWindows()) {
     oldmask = sys_umask(newmask);
   } else {
-    // TODO(jart): what should we do with this?
-    oldmask = newmask;
+    oldmask = atomic_exchange(&__umask, newmask);
   }
   STRACE("umask(%#o) → %#o", oldmask);
   return oldmask;