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/stdin-init.S

@@ -30,8 +30,5 @@
 	ezlea	__stdin_buf,cx
 	mov	%rcx,24(%rax)			#→ f.buf
 	movl	$BUFSIZ,32(%rax)		#→ f.size
-	ezlea	__freadbuf,cx
-	mov	%rcx,%rdx
-	mov	%rdx,40(%rax)			#→ f.reader
 	mov	%rax,stdin(%rip)
 	.init.end 400,_init_stdin,globl,hidden