Use vsscanf instead of rolling our own.

PR:		bin/140530
Submitted by:	Jeremy Huddleston <jeremyhu apple.com>

__sFILE.  This was supposed to be done in 6.0.  Some notes:
- Where possible I restored the various lines to their pre-__sFILEX state.
- Retire INITEXTRA() and just initialize the wchar bits (orientation and
  mbstate) explicitly instead.  The various places that used INITEXTRA
  didn't need the locking fields or _up initialized.  (Some places needed
  _up to exist and not be off the end of a NULL or garbage pointer, but
  they didn't require it to be initialized to a specific value.)
- For now, stdio.h "knows" that pthread_t is a 'struct pthread *' to
  avoid namespace pollution of including all the pthread types in stdio.h.
  Once we remove all the inlines and make __sFILE private it can go back
  to using pthread_t, etc.
- This does not remove any of the inlines currently and does not change
  any of the public ABI of 'FILE'.

MFC after:	1 month
Reviewed by:	peter

clause.

# If I've done so improperly on a file, please let me know.

Submitted by: keramida

I believe have made all of libc .c's as consistent as possible.

The change to reuse _up from FILE (to allow FILE to grow without changing
size) overlooked FILE being allocated off the stack.

Approved by:	sobomax