Stdio state variables.

The following always hold:

if (_flags&(__SLBF|__SWR)) == (__SLBF|__SWR), _lbfsize is -_bf._size, else _lbfsize is 0 if _flags&__SRD, _w is 0 if _flags&__SWR, _r is 0

This ensures that the getc and putc macros (or inline functions) never try to write or read from a file that is in `read' or `write' mode. (Moreover, they can, and do, automatically switch from read mode to write mode, and back, on "r+" and "w+" files.)

_lbfsize is used only to make the inline line-buffered output stream code as compact as possible.

_ub, _up, and _ur are used when ungetc() pushes back more characters than fit in the current _bf, or when ungetc() pushes back a character that does not match the previous one in _bf. When this happens, _ub._base becomes non-nil (i.e., a stream has ungetc() data iff _ub._base!=NULL) and _up and _ur save the current values of _p and _r.