buf: Rework xmlBuf code Always use what the old implementation called the "IO" allocation scheme, allowing to move the content pointer past the initial allocation. This is inexpensive and allows efficient shrinking. Optimize xmlBufGrow, reusing shrunken memory as much as possible. Simplify xmlBufAdd. Make xmlBufBackToBuffer return an error on overflow. Make "size" exclude the terminating NULL byte. Always provide an initial size. Reintroduce static buffers. Remove xmlBufResize and several other functions.