Improve Windows Console I/O

- Blocking read operations on the Windows Console can now EINTR
- Blocking read operations on Windows pipes now EINTR more reliably
- setitimer() will no longer be inherited across fork() on Windows
- It's now possible to use ECHO when the console is in raw mode
- The ECHOCTL flag now works correctly on the Windows Console
- The ICRNL flag now works correctly on the Windows Console
- pread() and pwrite() will now raise ESPIPE on Windows
- Opening /dev/tty on Windows is improved (untested)
- Overlapped I/O is now implemented in a better way

libc/stdio/fgetln.c

@@ -30,6 +30,9 @@
  * The returned memory is owned by the stream. It'll be reused when
  * fgetln() is called again. It's free()'d upon fclose() / fflush()
  *
+ * When reading from the console on Windows in `ICANON` mode, the
+ * returned line will end with `\r\n` rather than `\n`.
+ *
  * @param stream specifies non-null open input stream
  * @param len optionally receives byte length of line
  * @return nul-terminated line string, including the `\n` character