vfscanf: Use struct scratch_buffer instead of extend_alloca

vfscanf: Use struct scratch_buffer instead of extend_alloca

A custom character buffer is added in this commit, in the form of
struct char_buffer.  The char_buffer_add function replaces the
ADDW macro (which has grown with each successive security fix).
The char_buffer_add slow path is moved out-of-line, reducing
code size.

	* stdio-common/vfscanf.c (MEMCPY): Remove macro.
	(struct char_buffer): New type.
	(char_buffer_start, char_buffer_size, char_buffer_error)
	(char_buffer_rewind, char_buffer_add): New functions.
	(ADDW): Remove macro, replaced by the char_buffer_add function.
	(_IO_vfscanf_internal): Rewrite using struct char_buffer instead
	of extend_alloca.  Make control flow more explicit.