Support non-blocking i/o across platforms

This change introduces new tests for `O_NONBLOCK` and `SOCK_NONBLOCK` to
confirm that non-blocking i/o is now working on all supported platforms,
including Windows. For example, you can now say on Windows, MacOS, etc.:

    socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, IPPROTO_TCP);

To create a non-blocking IPv4 TCP socket. Or you can enable non-blocking
i/o on an existing socket / pipe / etc. file descriptor by calling fcntl

    fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | O_NONBLOCK);

This functionality is polyfilled on older Linux kernels too, e.g. RHEL5.
Now that fcntl() support is much better the FIOCLEX / FIONCLEX polyfills
for ioctl() have been removed since they're ugly non-POSIX diameond APIs

This change fixes a weakness in kprintf() that was causing Windows trace
tools to frequently crash.

libc/calls/readlinkat.c

@@ -56,6 +56,6 @@ ssize_t readlinkat(int dirfd, const char *path, char *buf, size_t bufsiz) {
     bytes = sys_readlinkat_nt(dirfd, path, buf, bufsiz);
   }
   STRACE("readlinkat(%s, %#s, [%#.*s]) → %d% m", DescribeDirfd(dirfd), path,
-         MAX(0, bytes), buf, bytes);
+         (int)MAX(0, bytes), buf, bytes);
   return bytes;
 }