Use XPath position() function instead of xsl:number, to evaluate array indices while generating code for easier readability.

API/ABI change: XCBSendRequest callers must pad to 4-byte boundaries now. When not in RAW mode, a null pointer for iov_base is replaced by up to 3 padding bytes.

There's no more race condition between event and reply handling.
The *RequestBlind and *RequestChecked functions are not yet implemented.