Put more thought into i/o polyfills

wait4() is now solid enough to run `make -j100` on Windows. You can now
use MSG_DONTWAIT on Windows. There was a handle leak in accept() that's
been fixed. Our WIN32 overlapped i/o code has been simplified. Priority
class now inherits into subprocesses, so the verynice command will work
and the signal mask will now be inherited by execve() and posix_spawn()

libc/calls/getcwd.greg.c

@@ -164,6 +164,6 @@ int __getcwd(char *buf, size_t size) {
   } else {
     rc = sys_getcwd_metal(buf, size);
   }
-  STRACE("__getcwd([%#hhs], %'zu) → %d% m", rc != -1 ? buf : "n/a", size, rc);
+  STRACE("getcwd([%#hhs], %'zu) → %d% m", rc != -1 ? buf : "n/a", size, rc);
   return rc;
 }