Make major improvements to stdio

Buffering now has optimal performance, bugs have been fixed, and some
missing apis have been introduced. This implementation is also now more
production worthy since it's less brittle now in terms of system errors.
That's going to help redbean since lua i/o is all based on stdio.

See #97

libc/stdio/putwchar.c

@@ -20,7 +20,8 @@
 
 /**
  * Writes wide character to stdout.
- *
  * @return wc if written or -1 w/ errno
  */
-wint_t putwchar(wchar_t wc) { return fputwc(wc, stdout); }
+wint_t putwchar(wchar_t wc) {
+  return fputwc(wc, stdout);
+}