Use SCANF_ISOC99_A instead of _IO_FLAGS2_SCANF_STD.

Change the callers of __vfscanf_internal and __vfwscanf_internal that
want C99-compliant behavior to communicate this via the new flags
argument, rather than setting bits on the FILE object.  This also
means these functions do not need to do their own locking.

Tested for powerpc and powerpc64le.